You weren't able to warp people using houses and 2P mounts in the first place. They fixes that glitch ages ago. If you invited someone to your 2P mount and left the house you would end up in different areas.
Horses are probably the buggiest thing in the game (no pun intended). Like Crag said, there could be just that one little-teensy-weensy tile that boots you off the horse completely wreck you out of nowhere. The castle was where the problem was because whenever you're in proximity you get kicked off. They really need to smoothen out the no mount zones. That's just ridiculous.
This doesn't solve problems, it creates new ones.