Sorry I looked different post and I thought you did dirty upgrade from 18 to 19. So many similar posts.Flashing the zip doesn't help. It was indeed no clean install. But several system flashes were fine, only this update from TWRP v3 to v4 leads to the data partition not be able to be mounted anymore even though I switched back. Did the new version change anything regarding partitioning? I want to avoid a reset and clean install now as much as possible :-|
Twrp by reingz v4 changelog is that support Android 13. I don't know if anything else changed, but I personally upgrade to v4 fine and I am in crDroid which use LineageOS as base.
Are you sure you used v4 System ext version and not NSE?
You can try to ask @REIGNZ3 . I don't know what went wrong in ur case.