[Android Q&A, Help & Troubleshooting] footbook:
Hey... I'm doing some testing... Here's the steps I've done so far:

wipe / reformat all partitions

entirely re-flash the whole Stock ROM

reboot

completed the Startup Wizard

set a PIN when requested (instead of a Password or a Pattern)

boot into Android

reboot

boot again into Android

reboot to Bootloader

fastboot flash recovery twrp.img

reboot into Android

reboot into TWRP

[V] Never show this screen during boot again

Swipe to Allow Modifications

TWRP is NOT asking me the PIN to decrypt /data

/data is NOT mounted, but with different error now (see logs down below)

every time I boot into TWRP, it always keeps me asking Swipe to Allow Modifications!!!

recovery.log

[...]

Data successfully decrypted, new block device: '/dev/block/dm-0'
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
Updating partition details...
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Unable to mount '/usb-otg'
I:Actual block device: '', current file system: 'vfat'
...done
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
Unable to mount storage
/data | /dev/block/dm-0 | Size: 0MB Used: 0MB Free: 0MB Backup Size: 0MB
Flags: Can_Be_Mounted Can_Be_Wiped Can_Be_Backed_Up Wipe_During_Factory_Reset Wipe_Available_in_GUI IsPresent Can_Be_Encrypted Is_Encrypted Is_Decrypted Has_Data_Media Can_Encrypt_Backup Use_Userdata_Encryption Is_Storage Is_Settings_Storage
Symlink_Path: /data/media
Symlink_Mount_Point: /sdcard
Primary_Block_Device: /dev/block/mmcblk0p49
Decrypted_Block_Device: /dev/block/dm-0
Crypto_Key_Location: footer
Length: -16384
Display_Name: data
Storage_Name: Internal Storage
Backup_Path: /data
Backup_Name: data
Backup_Display_Name: Data
Storage_Path: /data/media
Current_File_System: ext4
Fstab_File_System: ext4
Backup_Method: files
MTP_Storage_ID: 65537

I:Unmounting main partitions...
Successfully decrypted with default password.
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
Updating partition details...
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Unable to mount '/usb-otg'
I:Actual block device: '', current file system: 'vfat'
...done
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
Unable to mount storage
I:Unmounting main partitions...

[...]

I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
Failed to mount '/data' (No such process)
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Unable to mount /data/media during GUI startup.

[...]

I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Copying file /cache/recovery/log to /cache/recovery/last_log
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
Unable to mount /data/media/TWRP/.twrps
I:Attempt to load settings from settings file...
I:InfoManager file '/data/media/TWRP/.twrps' not found.
I:Backup folder set to '/data/media/TWRP/BACKUPS/WF012646'
I:Copying file /etc/recovery.fstab to /cache/recovery/recovery.fstab
I:Version number saved to '/cache/recovery/.version'
I:Unable to mount '/data'
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:Unable to mount '/usb-otg'
I:Actual block device: '', current file system: 'vfat'
I:TWFunc::Set_Brightness: Setting brightness control to 128
I:PageManager::LoadFileToBuffer loading filename: '/twres/languages/en.xml' directly
parsing languageFile
parsing languageFile done
I:Translating partition display names

[...]

Failed to mount '/data' (No such process)
I:Actual block device: '/dev/block/dm-0', current file system: 'ext4'
I:TWFunc::Set_Brightness: Setting brightness control to 5
I:TWFunc::Set_Brightness: Setting brightness control to 0

~ # dmesg

[...]

[ 5.628850] SELinux: initialized (dev mmcblk0p25, type ext4), uses xattr
[ 5.658433] EXT4-fs (mmcblk0p49): VFS: Can't find ext4 filesystem
[ 5.658777] EXT4-fs (mmcblk0p49): VFS: Can't find ext4 filesystem
[ 5.667880] EXT4-fs (mmcblk0p49): VFS: Can't find ext4 filesystem
[ 5.668184] EXT4-fs (mmcblk0p49): VFS: Can't find ext4 filesystem

[...]

[ 8.417484] device-mapper: req-crypt: req_crypt_ctr: Mapping block_device /dev/block/mmcblk0p49 to dm-req-crypt ok!

[...]

[ 14.258270] EXT4-fs (dm-0): warning: maximal mount count reached, running e2fsck is recommended
[ 14.259729] EXT4-fs warning (device dm-0): ext4_enable_quotas:5282: Failed to enable quota tracking (type=0, err=-3). Please run e2fsck to fix.
[ 14.259840] EXT4-fs (dm-0): mount failed
[ 14.261825] EXT4-fs (dm-0): warning: maximal mount count reached, running e2fsck is recommended
[ 14.263192] EXT4-fs warning (device dm-0): ext4_enable_quotas:5282: Failed to enable quota tracking (type=0, err=-3). Please run e2fsck to fix.
[ 14.263286] EXT4-fs (dm-0): mount failed

[...]

[ 56.822575] EXT4-fs (dm-0): warning: maximal mount count reached, running e2fsck is recommended
[ 56.824051] EXT4-fs warning (device dm-0): ext4_enable_quotas:5282: Failed to enable quota tracking (type=0, err=-3). Please run e2fsck to fix.
[ 56.824153] EXT4-fs (dm-0): mount failed
[ 56.826017] EXT4-fs (dm-0): warning: maximal mount count reached, running e2fsck is recommended
[ 56.827383] EXT4-fs warning (device dm-0): ext4_enable_quotas:5282: Failed to enable quota tracking (type=0, err=-3). Please run e2fsck to fix.
[ 56.827477] EXT4-fs (dm-0): mount failed

[...]

~ # e2fsck /dev/block/dm-0

e2fsck 1.42.9 (28-Dec-2013)
/dev/block/dm-0 has unsupported feature(s): quota
e2fsck: Get a newer version of e2fsck!