I don't think this is a bug as much as I think it's something the devs didn't implement, but I thought I should put it out there just in case it is a bug. Bats only get their AI overwritten by riding mobs if the bat is in the first air block, above a solid block. If the bat is any higher, then it's AI is not overwritten by the mob that rides it. It could probably be because some mobs are not scripted to go up and down like bats or blazes (didn't test the blaze tho).