Nitwits will not change career

I transformed a zombie villager to a villager and the villager's profession was a nitwit. I tried to change the villager to a librarian in the new beta 1.16 snapshot. The villager would not turn to a librarian, and I didn't trade with him at all. I tried with two other villagers and they turned, but when I removed the lectern they wouldn't change back, I never traded with them either.