I can confirm that it is still happening in the latest version. The steps to reproduce don't always lead to a disappearance, but I just had it happen when I tried to jump over a river with my horse, the horse appeared to land in an area that was 2 water blocks deep, and the horse vanished as I was forcefully dismounted. 1.17.11, Switch.
12 year old game is broken
I can confirm that it is still happening in the latest version. The steps to reproduce don't always lead to a disappearance, but I just had it happen when I tried to jump over a river with my horse, the horse appeared to land in an area that was 2 water blocks deep, and the horse vanished as I was forcefully dismounted. 1.17.11, Switch.