This bug was introduced in 1.21.4 Pre-Release 3.
Steps to reproduce (a video is attached):

1. Begin riding a horse (I used a tamed one).
2. Use the teleport command to teleport far away (I tested the bug by teleporting to 100000 100 100000).
3. The game gets softlocked in a state where the player cannot interact with entities, dismount the horse or execute commands (likely a complete softlock of the server); if the world is saved, one often gets stuck on the loading screen for a long time.
   When the world is reentered, it is to be noted that no progress has actually been saved.

Also note that, in a similar fashion to MC-278708, if one teleports to a location in close proximity, the horse is only dismounted and the teleport does not actually happen, but there is also no softlock.

Edit: Apparently, the softlock seems to resolve itself after a considerable amount of time (see attached image).