This has actually happened to me a couple times. The first time it happened was after I got onto my 1.16 world after updating it to 1.17. I noticed that a lot of my villagers were missing or had the wrong trades. I was upset, but pinned it on the new update and moved on and made a second villager farm. This time, when I was playing today, I was just running around my world and I noticed that my some of my villagers had disappeared. All of my librarian villagers and one put of my three farmer villagers to be specific. Thankfully, as far as I can tell, I got all of the enchantment books I needed out of them, but it is still very disappointing. I spent a lot of time getting the right enchantments from them and it's disappointing to know that I will have to start over again. I'm not expecting you to replace my villagers, but I would really appreciate if you fix this bug so that this doesn't happen a third time.