I created a Library and as Villagers were lured into it, I confined them and gave them a Lectern. They became Librarians and I traded with several of them (5). Later,

I released them from confinement, took down the Lecterns, and set them free. 8 remained Librarians. I learned that once traded with Villagers do not change professions or Trades.

I got them in a boat one at the time and smothered them by putting their heads inside a block; No Librarians as long as the door to the 'Library' was blocked. (No Lecterns, empty room)

The "smothering cycle" has been repeated several times and each time; when unemployed Villagers are allowed into the 'Library Building' they instantly become Librarians upon entering the room. If I block entrance to the building they wander around the Village as Librarians, even over several days.

I've upgraded from 1.16.2 to 1.17.1, removed the door to the room, removed the oak slab floor.

I spoke to Bozo A. in Support chat and he suggested upgrading and if unsuccessful to report it as a bug.