I don't know if this is the only case, but in this particular case, Horsea/Seadra should be able to learn Bubblebeam via TM in Gen I and get traded to a Gen II title. PKHeX doesn't seem to like it when the Pokémon is at it's final stage with the move. This is pretty minor and I haven't seen any other issues, but I thought I'd pop on here to see if there are, or if it had already been added to the latest commit.