This happens with any controllable mounts. You can see this by leashing a saddled pig to a happy ghast and riding the pig. If you don’t hold a carrot on a stick, it’ll work fine. If you do hold a carrot on a stick, the pig will fall down. It’s caused by the game prioritizing the player’s control of the mount over the happy ghast that it’s leashed to, which can be fixed by making it the other way around.
@brickticks You can effectively do the same thing with unsaddled camels