I noticed this after curing a zombie villager.

The villager had a luck of sea III book for sale. He then ran through the a portal in the room and when i went to get him on the other side he was now selling a fire aspect I. I managed to get him to go through the portal again and it again changed. I also noticed it reset the levels. 

PS: There is also a problem with trading and getting the villager to level but i saw that already in list.