Villager changes trades, after I've already traded with them

So, I decided to get some villagers, specifically librarians to get useful enchanted books. I broke and replaced the lectern to get the book I needed, as you normally would. I decided to lock in the villagers trades, so I bought a protection 4 book from the villager, this way the villager would not change his trades. Couple days later, I got online and I notices the villager was missing, and in his place was a villager who traded for a bookshelf, instead of prot 4. I could see by his experience bar that I had already traded with him, but I had never bought a bookshelf. I had never bought a bookshelf, but his experience bar suggest I have done so. The only explanation is that he changed his trades, even though I had already traded with him, which is not supposed to happen. later the exact same thing happened to a fortune 3 villager! And since I had "already traded with him", I couldn't change it back to the enchanted book I wanted. I didn't take a picture of the villager before he changed his trades, so I don't think a picture is applicable.

P.S. this happened on a realm