The hill just before the town in the Temple of Sun world has collision issues which prevents traversal up the slope, mounted or dismounted.
While one gets dismounted when interacting with a vendor, quest NPC, or similar the UI state and game state are not updated (just as on live). Ideally when getting dismounted in this fashion the game state would mirror this such that the first press of the mount key would start summoning the mount rather than "dismounting" from a no longer existing mount.