Villager changing job after trading

I'm playing on win 10. I cured a zombie villager inside a snow hut. It comes with the butcher job, and I tried to convert him to a librarian by letting it walk inside the basement and placing a bed and a lecturn. It didn't change the job though, so I started trading with it until it reaches level 3 (journeyman). And then its job magically changed to a librarian. idk why I can't select v1.19.30, as that's the version I'm playing on, anyway I just chose 1.19.31 instead.