Summary:
Uncontrolled mobs (horses (except for a horse skeleton), llama, camels,) can cut off leashes if a player will sit on them.

Steps to reproduce:

1. Summon a mob on which you can sit without driving it.
2. Leash the mob to the fence with a leash.
3. Sit on a mob.
4. Wait until the mob moves away far from the fence.

Observed results:
The mob does not tie the leash (he walks arbitrarily).

Expected results:
The leash will "tie" a mob even when a player sits on it.

Video:
Minecraft 2022-10-24 02-17-06.mp4