okay i have been playing around with this since i reported the bug and it seems that you just dismount on the southeast side of the animal but if there is a fence there you either end up on top of it or on the other side of it.

i thought this was a bug since i was unable to reproduce it at first with the horse and donkey and thought you just forgot the mule but they were just in the northwest corner of my stable so its just an annoyance not exactly a bug