[OnePlus 3 & 3T Cross Device Development] Doc Sniper:
Tried TWRP blu_spark v8.61 and it was working fine, like the "old" nougat based TWRP 3.2.1-0.

In the log the file system is now detected as f2fs, but I wonder why because on my other 3T the DATA partition is also formatted f2fs and the "new" oreo based 3.2.1 detects ext4 and everything is working (see the log below).

BlueSpark TWRP on the problem phone:

[...]
I:Done processing fstab files
I:Setting up '/data' as data/media emulated storage.
I:Can't probe device /dev/block/sda15
I:Unable to mount '/data'
I:Actual block device: '/dev/block/sda15', current file system: 'f2fs'
I:Can't probe device /dev/block/sda15
I:Unable to mount '/data'
I:Actual block device: '/dev/block/sda15', current file system: 'f2fs'
get_crypt_ftr_info crypto key location: 'footer'
I:Backup folder set to '/data/media/TWRP/BACKUPS/12345678'
I:Settings storage is '/data/media'
I:Device is encrypted with the default password, attempting to decrypt.
crypt_ftr->fs_size = 237437880
Using scrypt with keymaster for cryptfs KDF
Invalid hex string
Failed to convert passwd from hex, using passwd instead
keymaster module name is Keymaster QTI HAL
keymaster version is 256
Found keymaster1 module, using keymaster1 API.
Signing safely-padded object
Using scrypt with keymaster for cryptfs KDF
Invalid hex string
Failed to convert passwd from hex, using passwd instead
keymaster module name is Keymaster QTI HAL
keymaster version is 256
Found keymaster1 module, using keymaster1 API.
Signing safely-padded object
crypt_ftr->fs_size = 237437880
Using scrypt with keymaster for cryptfs KDF
keymaster module name is Keymaster QTI HAL
keymaster version is 256
Found keymaster1 module, using keymaster1 API.
Signing safely-padded object
Using scrypt with keymaster for cryptfs KDF
keymaster module name is Keymaster QTI HAL
keymaster version is 256
Found keymaster1 module, using keymaster1 API.
Signing safely-padded object
load_crypto_mapping_table: target_type = req-crypt
load_crypto_mapping_table: real_blk_name = /dev/block/sda15, extra_params = fde_enabled ice
Password did not match but decrypted drive mounted - continue
Data successfully decrypted, new block device: '/dev/block/dm-0'
I:mount -o bind '/data/media' '/sdcard' process ended with RC=0
I:Backup folder set to '/data/media/0/TWRP/BACKUPS/12345678'
Updating partition details...
[...]

The "new" oreo based TWRP 3.2.1-0 on the working phone:

[...]
I:Done processing fstab files
I:Setting up '/data' as data/media emulated storage.
I:Can't probe device /dev/block/sda15
I:Unable to mount '/data'
I:Actual block device: '/dev/block/sda15', current file system: 'ext4'
I:Can't probe device /dev/block/sda15
I:Unable to mount '/data'
I:Actual block device: '/dev/block/sda15', current file system: 'ext4'
get_crypt_ftr_info crypto key location: 'footer'
I:Backup folder set to '/data/media/TWRP/BACKUPS/87654321'
I:Settings storage is '/data/media'
I:Device is encrypted with the default password, attempting to decrypt.
crypt_ftr->fs_size = 237437880
Using scrypt with keymaster for cryptfs KDF
Trying to convert ascii string of odd length
Failed to convert passwd from hex, using passwd instead
keymaster module name is Keymaster QTI HAL
keymaster version is 256
Found keymaster1 module, using keymaster1 API.
Signing safely-padded object
Using scrypt with keymaster for cryptfs KDF
Trying to convert ascii string of odd length
Failed to convert passwd from hex, using passwd instead
keymaster module name is Keymaster QTI HAL
keymaster version is 256
Found keymaster1 module, using keymaster1 API.
Signing safely-padded object
crypt_ftr->fs_size = 237437880
Using scrypt with keymaster for cryptfs KDF
keymaster module name is Keymaster QTI HAL
keymaster version is 256
Found keymaster1 module, using keymaster1 API.
Signing safely-padded object
Using scrypt with keymaster for cryptfs KDF
keymaster module name is Keymaster QTI HAL
keymaster version is 256
Found keymaster1 module, using keymaster1 API.
Signing safely-padded object
load_crypto_mapping_table: target_type = req-crypt
load_crypto_mapping_table: real_blk_name = /dev/block/sda15, extra_params = fde_enabled ice
I:Found no matching fstab entry for uevent device '/devices/virtual/block/dm-0' - add
I:Found no matching fstab entry for uevent device '/devices/virtual/block/dm-0' - change
Password did not match but decrypted drive mounted - continue
Data successfully decrypted, new block device: '/dev/block/dm-0'
I:mount -o bind '/data/media' '/sdcard' process ended with RC=0
I:Backup folder set to '/data/media/0/TWRP/BACKUPS/87654321'
Updating partition details...
[...]