TWRP is not compatible with Link2SD, therefore one must manually mount MicroSD Card 2nd partition each time before TWRP backup/restore is started.
One can do this from - TWRP menu - Advanced - Terminal
mount /dev/block/mmcblk1p2 /data/sdext2
TWRP backup will include 2nd partition in data.ext4.win as regular directory sdext2 therefore it is important to ...
Failure [INSTALL_FAILED_INVALID_INSTALL_LOCATION] indicates something is wrong with signature.
If app is moved to MicroSD Card primary partition, Android native app2sd - settings - app - storage - MOVE TO SD CARD - created encrypted container in .android_secure directory which is mounted to /mnt/secure/asec
The same MicroSD Card cannot used in another phone ...