Cataclysm: This toon and all of my alts.
I suppose zoning in to Mount Hyjal would be one way to reproduce the issue but it’s happened other times while not in Hyjal. The way it was explained to me in-game is that we’re being phased from out own mount just like we are when we’re forced to play Horde in PVP as an Alliance member.
I don’t know if this is working as intended but I do know it’s an expensive mount that I put a lot of effort into obtaining. I don’t ever remember this happening in original Cataclysm and am hoping this is just a bug that will be fixed very soon.