Interestingly, I attempted to trade on this game mode and to my surprise, it succeeded! I believe this has to do with the fact that they are alternate forms, which makes it technically compatible with the base game! E.g In base game, Pokemon form 0 and 1 are the same unless Alolan. Basically it means that pokemon traded from Alternate forms to normal reborn will appear as normal, rather than alternate forms, although with a questionable moveset.