If you try locking in a villager by doing a single trade then break workstation they turn back into brown coats, then will relink if you put workstation back down. After relinking the villager will be automatically locked into whatever random trades generated when they relinked. Sometimes it doesn't even matter if you level it to apprentice+, it still resets. Had one with looting3 leveled up to apprentice and went today to get it once I had emeralds, and he was a bookshelf trader that was already leveled to apprentice.