Maybe this bug is related to MC-47508 and/or MC-23320.

In singleplayer I sometimes leave tamed horses and donkeys behind in the wild to pick them up later if I happen upon a donkey or horse that I'd rather take home first.

So in this case, I left a previously tamed and ridden donkey (but without chest) on the map near a herd of grazing horses, because I wanted to take one of the horses back to my other horses for breeding.
When I returned to the place where I had left the donkey after a few Minecraft days, it was untamed (gone wild again?) and it got new health. Before I left it, it had had 15 hearts, when I returned to it, there were only 11 (and there wasn't any damage, I mean the new maximum was 11).

I would have expected the donkey to stay tamed even if I left it for some days and the hearts to remain the same at least. If this reset of the stats happens to animals in the wild I'm wondering whether it would be possible to tame wild donkeys and horses (even if they didn't have a lot of hearts), leave them in the wild and come back some time later. Maybe that would reset them to different stats?