In the search to find a mending trade, I expanded my village to 32 villagers through breeding. Then placed 11 lecterns to provide a profession for the nitwits, trying to find a mending trade. I kept breaking the lectern and replacing it to reset their trades, and this mostly worked at the right time of day, as normal. However when i did finally get my mending villager, I stopped destroying lecterns and locked the mending villager away so he can't run off. When I was working on creating a separate home for him, he suddenly started to glitch back and forth between nitwit and librarian. When the glitching was over, his trades had changed to something else again, and I lost the mending trade.

The same thing happened to other villagers, and my theory was that it's because there were too many villagers and other nitwits around and they were trying to steal each other's professions. That, or it's just a glitch. It has only happened to me in this situation, not before that. I tried again in a creative file and it happened again, I didn't have quite as many villagers then but I now theorise it's the amount of lecterns that were placed that caused the bug. I haven't tried it with other professions.

For more specifics it was a desert village, pretty small initially but I made new homes and beds to provide for the newly bred villagers. The bug happened most in an area further away from the village centre but still technically within the border (there was still a naturally generated home next to it, actually even further away from the centre.)