I had 2 tamed horses attached to a fence with leads.
After exiting and restarting the game multiple times the horses where no longer there. One horse was far away, the other horse close to the fence, with a lead item placed on the ground.
I tried to reproduce this, which I only managed once when I was far away from the fence to just be visible.

Could this be caused by the horse and the attached lead being placed in different junks (similar to MCPE-11477) so that not both ends of the lead are loaded?