I've only ever experienced that bug after training on a char for awhile, using necro forms and spamming them for skill. The last time it happened I tried to page a GM to show the bug...forget what the result of that was, though, whether the GM said to submit it to bug report or what, but I don't remember being able to chat with a GM or show them the bug as it happened. But it's a pretty old bug, nothing new, whatever it is.
Anyway, this was taken in May:
Basically if you dismount, the mount auto-stables. You go to the stable, mount it, unmount, it auto-stables again. Not sure why it happens, but it doesn't happen on this particular char now so it is self-fixable or corrected by something.