I recently had a new world with a relatively small stable (about 15 boxes). Horses keep disappearing from this stable, so only the horses, not the donkeys.
The boxes are big enough, the stall is made of sandstone with wooden gates as doors. They also have a small paddock. In principle everything is the same as in my other worlds and the horses are still alive even though I don't ride most of them anymore or don't look after them.
For comparison: I go into this world almost every day and every time at least 1 horse is missing. In another world with a huge stable with approx. 80 boxes, only 4 are missing, although I haven't looked at those at the top for ages.
I already fed the horses yesterday, but some of them are already gone. I really have no idea what else to do. The biggest difference is that the world is a lowland world and the others are not, but it can't be because of that.
Try to tame the horses and you have to consider that if you have too many horses inside, they will automatically disappear 🤷🏼♀️
They are all tamed and have saddles on them and everything. Would you build several stables then?