So I created a villager trading hall. I grouped all villagers in one large room and put a lectern in a hall for each villager. During the "work period" the villagers automatically walk into the hall with their profession block on their own. I trap each villager in their specific hall and all is fine. 

I begin re-rolling the first villager's trades until I get a Mending enchantment option. Once I get the Mending option, I walk over to my chest at the other side of the room to get my emeralds, walk back, and suddenly the trade has disappeared and been replaced with something new, without me even breaking the profession block again!

I have tried just about everything to stop them from auto-rerolling their trades but nothing has worked. I don't want to "lock in" a trade right away either as I may get a better cost from another one of my villagers but it looks like it will be the only option if this bug isn't resolved soon. Any ideas?