I mounted my donkey and traveled west for 1200+ blocks, and upon dismounting, the donkey disappears and is nowhere to be found. I can still hear his snorts, however, which is strange.
Same thing happens when I got a horse. Traveling north this time, I dismount and the horse disappears, with only his snorts being heard.
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From personal experience I have found this bug to extend to all tamed animals
I highly doubt that this is MC-96954.
The bug he describes has nothing to do with dismounted Entities floating somewhere in the air. It is more like the mounted entity is desynced, but not the riding one.
I know that even when traveling short distances in a boat, when exiting it, the boat might disappear and shortly "move" back in (as if it got teleported). I think that this is the bug he describes.
I cannot find a bug that describes that, but it is probably somewhere.