Villager trade price increase without apparent reason

NOT A DUPLICATE

I had an amazing villager, a cured one. He traded 1 emerald (and a book) for a sharpness V book. 

He suddenly increased his trade price to 2 emeralds, but I can't find any reason why. I didn't hit or kill any villagers near him or anything of the sort. Nor did I heavily affect the supply/demand system, as I only bought the book twice. I've bought more mending books for 1 emerald and that price is still the same...

This happened again with a depth strider III villager (also 1 emerald). Except in this case, he increased it from 1 to 2 and, less than a minute or two later, to 3. 

I find this very weird... I have mending, fortune III and unbreaking III villagers who haven't changed their prices at all, even though they have been in the world for a longer time and I have traded with the first and last one more.

The reason I believe this is not a duplicate is because the previous ones were all dismissed by saying "You must've hurt a villager", "You probably bought him out", "You waited too long, the effect faded" (I don't think the major positive effect from cured villagers fades...), etc. I did nothing of the sort. (Didn't buy more than twice).

Thank you for reading through my issue 🙂