After dying while riding a horse a large enough distance away from your respawn point, the horse will disappear after reloading the world if it wasn't loaded after you respawned.

To reproduce

1. Set your spawn somewhere
2. Teleport ~1000 blocks away
3. Summon a horse

   /summon horse ~ ~ ~ {Tame:1b}

4. Ride the horse
5. Summon a zombified piglin (or another mob)
6. Let it kill you
7. When back at your respawn point, reload the world
8. Teleport back
   → The horse disappears

Video

Minecraft 21w05b - Singleplayer 2021-02-10 13-31-58_Trim.mp4

Original description

I was riding my horse when I was killed by an enderman. I was heading back to where I died and for some reason I was somehow teleported onto my horse. The game glitched me onto the horse. After I was glitched onto the horse, no sounds could be made. The horse couldn't make sound, but other animals were still able to move but couldn't make sound. Chunks were still rendering though. I was looking at a map when the game glitched. When I was on the glitched horse, the map thought I was in the same position as I was before the game glitched. When I reloaded my world, I was in the same area as the horse, but the horse was gone. I tried using commands to find the horse, but it couldn't find it. PLEASE FIX THIS