Can't remember what version twrp was but using twrp earlier than Oreo releases will result in mounting data issues due to the new data management introduce in Oreo.
Just curious are you copy backups file from external to internal storage. Unlike flashing, twrp has a dedicated storage location for internal (and external ) backups.
I backup to internal (then have to move to external to see it when restoring back up), when I try to restore backup, it shows no folders or anything at all internally, but if Im going to back up or select zip to flash, it shows folders and such. So even when I backup internally, then hit restore, nothing is there, it does show how much storage is left internally