He actually wasn't terrible against LHP in the minors. But his slugging disappeared. He got on base a lot.
That said, Busch changed his swing this year. He's essentially geared it to hitting RHP, which is fine. I don't think he was ever going to be a good hitting 1b against LHP - so they said "horsefeathers it, just crush RHP and well find someone to play against lefties". Meaning I'm not sure his Triple-A splits matter much any more, either.