Dismounting a horse inside a portal sometimes does not cause it to go through

To reproduce:

1. Mount a horse (preferably tamed).
2. Go into a portal. I was only able to reproduce it with Nether portals.
3. Dismount the horse.

Result: Sometimes the horse goes through the portal, sometimes it doesn't. I wasn't able to find a clear pattern. Pushing the horse around sometimes seems to help.