I may have made a mistake here. I have now found out that there was also a herd of wild donkeys near the place where I left my tamed 15 hearts donkey. One of them seems to have wandered over to the place where I left "my" donkey (about 150 blocks distance) and "my" donkey has wandered off another 100 blocks in a different direction. Unless someone else has experienced the same issue, I think this could be closed. I will continue to try leaving tamed horses and donkeys (that I don't want for breeding any more) in the wild and see how they behave, but this one checks out now.
I may have made a mistake here. I have now found out that there was also a herd of wild donkeys near the place where I left my tamed 15 hearts donkey. One of them seems to have wandered over to the place where I left "my" donkey (about 150 blocks distance) and "my" donkey has wandered off another 100 blocks in a different direction.
Unless someone else has experienced the same issue, I think this could be closed.
I will continue to try leaving tamed horses and donkeys (that I don't want for breeding any more) in the wild and see how they behave, but this one checks out now.