I have a villager breeder which sends all babies into the nether. Once there, I would like to be able to set their profession and lock it in, otherwise they'll all be fishermen due to the fact I use barrels for storage. However, they will not take any profession in the nether unless they already have one, in which case they will not change even though they haven't been trades with. I could understand if it were perhaps "fear" of other mobs, but they are completely enclosed in a safe room where no mobs can spawn.