Riding a mount, such as a horse, when the horse is teleported to a region unrendered by players causes the player to appear to be dismounted. The player can move away from the mount but will continuously be teleported back to the mount until they sneak to get off of it.

To test:
1. Summon tame and saddled horse
2. Ride horse
3. Teleport horse 10 blocks away
4. Notice still riding horse fine
5. Teleport horse 100 blocks away
6. Notice "dismount" glitch
7. Sneak to end glitch
8. Kill horse to notice horse does not visually die
9. Teleport -100 blocks back to where you were before to see item drops from the dead horse. Horse at 100 blocks other way is no longer there visually or physically because unloaded

For step 3. /spreadplayers will also cause this if the distance from original position is far enough