I have found this same problem. My horse seems to be fine dismounting near where I mounted it, but I found that when trying to dismount in any chunks that are not loaded when first mounting the horse, it disappears like for everyone else. This also happens after visiting an unloaded chunk and returning to a chunk that was loaded when mounting the horse. I hope this helps (although I may be a bit late, it might help if the current solution doesn't work)
Adrian Leighton
I have found this same problem. My horse seems to be fine dismounting near where I mounted it, but I found that when trying to dismount in any chunks that are not loaded when first mounting the horse, it disappears like for everyone else. This also happens after visiting an unloaded chunk and returning to a chunk that was loaded when mounting the horse. I hope this helps (although I may be a bit late, it might help if the current solution doesn't work)