Noticed this on a SMP server. When a leashed baby horse grows from "newborn" to "pre-adult" it seems to ignore the fact its on a leash. Meaning the leash doesn't even try to pull it back, but it still holds the limit point so that when the horse walks past the limit point the leash breaks. If you push (not hit) the horse before it breaks the leash, everything seems to re-establish and the leash pulls the horse again, until it grows again. Once its fully grown the leash seems to be fine its just in those in between points when the Baby horse isn't full grown.
Don't believe the server I play on is modded so I will attempt to reproduce on SSP when I can.
Also I didnt see any bugs relating to this when I searched but the search has been spammed with dupicated posts of suffocating horses and breaking leashs due to TP its hard to sort through them all.