I built an enclosed building trading hall, in an fenced off village, in close proximity to my home base. I had a Mending librarian on the 2nd level of this floor, down the hall from the ladder. Each villager was enclosed in a 1x1 square, blocked off with glass on the sides, and wood on the bottom, and their respective trading podium in front leaving a 1x1 block to access trades.

I was derping about my base, and wanted to get a new mending book, so I went to my trading hall–but the Mending villager was gone. He was not in the village anywhere, he was not in his cell, there was no Zombie/hostile mob in the building. He was not name tagged, but was either gold or Silver level traded with.

This occurred previously on 1.18, but I had assumed the villager (as he was loose in the village) had just died but now I am not certain.