[Magisk] _Tille_:
[MENTION=5228676]VR25[/MENTION], thank you! fbind -f resolved it. A bit strange, the SDcard was not changed during the upgrade to Samsung Oreo. Below are current outputs of the boot log and /proc/mounts.

Boot Log

Sun Jan 6 07:51:21 CET 2019
versionCode=201901050
Device=a3y17lte

+ . /sbin/.core/img/fbind/core.sh
+ altExtsd=false
+ tmpf=/dev/fbind/tmpf
+ intsd=/data/media/0
+ obb=/data/media/obb
+ modData=/data/adb/fbind
+ config=/data/adb/fbind/config.txt
+ alias 'mount=/sbin/su -Mc mount -o rw,noatime'
+ '[' -z ']'
+ interactiveMode=false
+ '[' -e /data/adb/fbind/.fuse ']'
+ touch /data/adb/fbind/.fuse
+ grep -q noAutoMount /data/adb/fbind/config.txt
+ apply_config
+ '[' -e /data/adb/fbind/config.txt ']'
+ grep -iq ^permissive /data/adb/fbind/config.txt
+ grep -v 'part ' /data/adb/fbind/config.txt
+ grep -q 'fsck.*mmcblk1'
+ set +x
+ '[' /data/adb/fbind/config.txt -nt /data/media/0/.fbind_config_backup.txt ']'
+ mkdir -p /dev/fbind
+ >/dev/fbind/tmpf
+ grep -E '^extsd_path |^intsd_path |^part |^loop ' /data/adb/fbind/config.txt
+ . /dev/fbind/tmpf
+ rm /dev/fbind/tmpf
+ false
+ default_extsd
+ typeset 'dir=' 'size=0' 'newSize=0'
+ false
+ wait_until_true grep -q /mnt/media_rw /proc/mounts
+ typeset 'count=0'
+ '[' 0 -ge 1800 ']'
+ count=2
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ sleep 2
+ '[' 2 -ge 1800 ']'
+ count=4
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ sleep 2
+ '[' 4 -ge 1800 ']'
+ count=6
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ sleep 2
+ '[' 6 -ge 1800 ']'
+ count=8
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ sleep 2
+ '[' 8 -ge 1800 ']'
+ count=10
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ sleep 2
+ '[' 10 -ge 1800 ']'
+ count=12
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ sleep 2
+ '[' 12 -ge 1800 ']'
+ count=14
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ sleep 2
+ '[' 14 -ge 1800 ']'
+ count=16
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ sleep 2
+ '[' 16 -ge 1800 ']'
+ count=18
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ sleep 2
+ '[' 18 -ge 1800 ']'
+ count=20
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ sleep 2
+ '[' 20 -ge 1800 ']'
+ count=22
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ break
+ grep -Eiq ' sdcardfs | fuse ' /proc/mounts
+ '[' -n grep ']'
+ grep -q /mnt/media_rw /proc/mounts
+ grep -q /mnt/media_rw /proc/mounts
+ '[' -e /mnt/media_rw/32F7-DF7F ']'
+ df /mnt/media_rw/32F7-DF7F
+ tail -n1
+ awk '{print $2}'
+ newSize=124863744
+ '[' 124863744 -gt 0 ']'
+ size=124863744
+ extsd=/mnt/media_rw/32F7-DF7F
+ '[' -e '/mnt/media_rw/.*' ']'
+ is_mounted /mnt/media_rw/32F7-DF7F
+ 2>/dev/null
+ mountpoint -q /mnt/media_rw/32F7-DF7F
+ extobb=/mnt/media_rw/32F7-DF7F/Android/obb
+ grep -iq sdcardfs /proc/mounts
+ grep -Eq '^int_extf|^bind_mount |^obb.*|^from_to |^target ' /data/adb/fbind/config.txt
+ bind_mount_wrapper
+ false
+ '[' -n ']'
+ >/dev/fbind/tmpf
+ grep -E '^int_extf|^bind_mount |^obb|^from_to |^target ' /data/adb/fbind/config.txt
+ . /dev/fbind/tmpf
+ from_to WhatsApp .fbind/WhatsApp
+ bind_mount /mnt/media_rw/32F7-DF7F/.fbind/WhatsApp /data/media/0/WhatsApp
+ is_mounted /data/media/0/WhatsApp
+ 2>/dev/null
+ mountpoint -q /data/media/0/WhatsApp
+ false
+ wait_storage /mnt/media_rw/32F7-DF7F/.fbind/WhatsApp /data/media/0/WhatsApp
+ echo /mnt/media_rw/32F7-DF7F/.fbind/WhatsApp /data/media/0/WhatsApp
+ grep -Eq '/data/media/[1-9]|/storage/emulated|/mnt/runtime/default/'
+ grep -q /mnt/media_rw/
+ echo /mnt/media_rw/32F7-DF7F/.fbind/WhatsApp /data/media/0/WhatsApp
+ wait_until_true grep -q /mnt/media_rw/ /proc/mounts
+ typeset 'count=0'
+ '[' 0 -ge 1800 ']'
+ count=2
+ '[' -n grep ']'
+ grep -q /mnt/media_rw/ /proc/mounts
+ break
+ grep -Eiq ' sdcardfs | fuse ' /proc/mounts
+ '[' -n grep ']'
+ grep -q /mnt/media_rw/ /proc/mounts
+ :
+ '[' 0 -ne 0 ']'
+ mkdir -p /mnt/media_rw/32F7-DF7F/.fbind/WhatsApp /data/media/0/WhatsApp
+ /sbin/su -Mc mount -o rw,noatime -o rbind '"/mnt/media_rw/32F7-DF7F/.fbind/WhatsApp"' '"/data/media/0/WhatsApp"'
+ grep -iq sdcardfs /proc/mounts
+ is_mounted /data/media/0/WhatsApp
+ 2>/dev/null
+ mountpoint -q /data/media/0/WhatsApp
+ '[' -z ']'
+ 2>/dev/null
+ rm -rf /mnt/media_rw/32F7-DF7F/.fbind/WhatsApp/Android
+ rm /dev/fbind/tmpf
+ false
+ grep -q '^remove ' /data/adb/fbind/config.txt
+ 2>/dev/null
+ rm /data/adb/fbind/.fuse
+ exit 0

Proc Mounts

rootfs / rootfs ro,seclabel,size=844908k,nr_inodes=211227 0 0
tmpfs /dev tmpfs rw,seclabel,nosuid,relatime,mode=755 0 0
devpts /dev/pts devpts rw,seclabel,relatime,mode=600 0 0
proc /proc proc rw,relatime,gid=3009,hidepid=2 0 0
sysfs /sys sysfs rw,seclabel,relatime 0 0
selinuxfs /sys/fs/selinux selinuxfs rw,relatime 0 0
/sys/kernel/debug /sys/kernel/debug debugfs rw,seclabel,relatime 0 0
none /acct cgroup rw,relatime,cpuacct 0 0
tmpfs /mnt tmpfs rw,seclabel,relatime,mode=755,gid=1000 0 0
none /config configfs rw,relatime 0 0
tmpfs /mnt/secure tmpfs rw,seclabel,relatime,mode=700 0 0
tmpfs /mnt/secure/asec tmpfs rw,seclabel,relatime,mode=700 0 0
none /dev/cpuctl cgroup rw,relatime,cpu 0 0
pstore /sys/fs/pstore pstore rw,seclabel,relatime 0 0
/dev/block/platform/13540000.dwmmc0/by-name/SYSTEM /system ext4 ro,seclabel,relatime,norecovery 0 0
/dev/block/platform/13540000.dwmmc0/by-name/CACHE /cache ext4 rw,seclabel,nosuid,nodev,noatime,journal_checksum,journal_async_commit,noauto_da_alloc 0 0
/dev/block/platform/13540000.dwmmc0/by-name/EFS /efs ext4 rw,seclabel,nosuid,nodev,noatime,discard,journal_checksum,journal_async_commit,noauto_da_alloc,data=ordered 0 0
/dev/block/platform/13540000.dwmmc0/by-name/CPEFS /cpefs ext4 ro,seclabel,nosuid,nodev,noatime,discard,journal_checksum,journal_async_commit,noauto_da_alloc,data=ordered 0 0
tmpfs /storage tmpfs rw,seclabel,relatime,mode=755,gid=1000 0 0
adb /dev/usb-ffs/adb functionfs rw,relatime 0 0
/dev/block/dm-0 /data ext4 rw,seclabel,nosuid,nodev,noatime,journal_checksum,journal_async_commit,noauto_da_alloc,resgid=5555 0 0
tmpfs /sbin tmpfs rw,seclabel,relatime 0 0
/dev/block/platform/13540000.dwmmc0/by-name/SYSTEM /sbin/.magisk/mirror/system ext4 ro,seclabel,relatime,norecovery 0 0
/dev/block/dm-0 /sbin/.magisk/mirror/bin ext4 rw,seclabel,nosuid,nodev,noatime,journal_checksum,journal_async_commit,noauto_da_alloc,resgid=5555 0 0
/sbin/.magisk/block/loop08 /sbin/.magisk/img ext4 rw,seclabel,noatime 0 0
/sbin/.magisk/block/loop08 /system/app/YouTube ext4 rw,seclabel,noatime 0 0
tmpfs /system/xbin tmpfs rw,seclabel,relatime 0 0
/sbin/.magisk/block/loop08 /system/xbin/fbind ext4 rw,seclabel,noatime 0 0
/dev/block/platform/13540000.dwmmc0/by-name/SYSTEM /system/xbin/cpustats ext4 ro,seclabel,relatime,norecovery 0 0
/dev/block/platform/13540000.dwmmc0/by-name/SYSTEM /system/xbin/jack_connect ext4 ro,seclabel,relatime,norecovery 0 0
/dev/block/platform/13540000.dwmmc0/by-name/SYSTEM /system/xbin/jack_disconnect ext4 ro,seclabel,relatime,norecovery 0 0
/dev/block/platform/13540000.dwmmc0/by-name/SYSTEM /system/xbin/jack_lsp ext4 ro,seclabel,relatime,norecovery 0 0
/dev/block/platform/13540000.dwmmc0/by-name/SYSTEM /system/xbin/jack_showtime ext4 ro,seclabel,relatime,norecovery 0 0
/dev/block/platform/13540000.dwmmc0/by-name/SYSTEM /system/xbin/jack_simple_client ext4 ro,seclabel,relatime,norecovery 0 0
/dev/block/platform/13540000.dwmmc0/by-name/SYSTEM /system/xbin/jack_transport ext4 ro,seclabel,relatime,norecovery 0 0
/dev/fuse /mnt/runtime/default/emulated fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/fuse /storage/emulated fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/fuse /mnt/runtime/read/emulated fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/fuse /mnt/runtime/write/emulated fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/block/vold/public:179,33 /mnt/media_rw/32F7-DF7F sdfat rw,dirsync,nosuid,nodev,noexec,noatime,nodiratime,fs=exfat,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,utf8,namecase=0,adj_req,symlink=0,bps=512,errors=remount-ro 0 0
/dev/block/vold/public:179,33 /mnt/secure/asec sdfat rw,dirsync,nosuid,nodev,noexec,noatime,nodiratime,fs=exfat,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,utf8,namecase=0,adj_req,symlink=0,bps=512,errors=remount-ro 0 0
/dev/fuse /mnt/runtime/default/32F7-DF7F fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/fuse /storage/32F7-DF7F fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/fuse /mnt/runtime/read/32F7-DF7F fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/fuse /mnt/runtime/write/32F7-DF7F fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/block/vold/public:179,33 /data/media/0/WhatsApp sdfat rw,dirsync,nosuid,nodev,noexec,noatime,nodiratime,fs=exfat,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,utf8,namecase=0,adj_req,symlink=0,bps=512,errors=remount-ro 0 0