I've setup a trading hall of librarians (originally in 1.14.2), with a sign above each librarian's lectern so that I could keep track of their trades. The librarians are allowed to mingle with the village and "gossip".
After the upgrade to 1.15.1 (and re-verified in a backup of the world that was opened in 1.14.4), leaving the village, and coming back into it later (far enough away for the village chunks to be unloaded, and then reloaded), I see the following:
- multiple "novice" librarians have reset their trades (and their progress bar back to 0), even ones that have had 1-2 trades. None of the "apprentice" level librarians have reset their trades
- Which librarian is using which lectern has been shuffled. They're consistently using a different lectern than their original one.
I've confirmed that the chunk(s) that contains my trading hall are different from the chunk that contains the village bell, and where the villagers congregate to gossip.
My guess is that when I'm leaving/entering the village, the chunk with the hall in it is unloaded, and the villagers can't find their job-sites, and then when the chunk with the job-sites is loaded, the villagers end up randomizing which jobsite is theirs.
This seems like it's similar to the following, perhaps with the same underlying cause: