If a horse is riding a mob, and that mob gets killed, players riding that horse get kicked off the horse client-side but not server-side.
To reproduce
Run the following command:
/summon husk ~ ~ ~ {Passengers:[{id:"minecraft:horse",Tame:1b}]}
Place a block of TNT next to the husk
Ignite that TNT
Mount the horse
When the husk dies, you get dismounted
Walk away
Sneak
→ ❌ You get teleported back to the horse
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Can confirm in 21w03a.

Can this still be reproduced in 21w05 or later?
This has been fixed in 21w05a, you no longer get dismounted from the horse at all when the husk dies.
Can confirm in 20w51a.