When I try to spawn a villager with custom trades and profession, the villager spawns with no profession at all. When I try to trade with it, it will sometimes deny the action and sometimes show the trading popup, but hide it immediately afterward. Once the villager gets a profession these problems are fixed, but the custom trades don't persist and new trades are generated. Interestingly, none of this appears to be a problem when I summon the villager with the "NoAI" tag set to true.