in 1.15.3 i found the bug but have waited for this snapshot and found it here also.

You can trade with a villager at daytime 3 a 4 times, then you hear the sound that he use his object to reset his trades, but his trade stays locked, after 3 times using his personal block you can trade then again 1 time and then the trade stays locked, but he use his object until it is villager sleep time, but i can't trade that much as you hear the sound that he is using his block. Librarian for example is looking at you, he is turning his page but after 3 times he stays locked but is still turning his page. I know that this is a bug because on multiplayer 1.14 you see the villager turn his page and after every sound he is turning his page the villager reset his trade. I hope i explained it enough. And i have tested it in both versions. In 1.14 you can trade until it is villager sleeping time.