ridden mobs will dissapear when dismounted over the top of a water, to be specific, donkey with chest and saddle

this happen 100%, and the ridden mob would sttill be there, you can still hear it.

the donkey is also still unnamed

donkey will dissapear upon dismount [shift] at the top of  the water, and by quitting and reentering the game world the donkey will reappear