Summary:
Caused by the fix of MC-191714 and introduced in 21w05a. If the farmland or dirt path that the entity you are controlling is standing on converts to dirt, it causes you and the controlled entity to glitch and behave as if it is falling. This means that boats will break, mobs will take fall damage and you will take fall damage as well.

Steps to reproduce:

1. Place a farmland block on the ground
2. Tame a horse and saddle it
3. Jump with the horse on the farmland
4. Go into survival mode

Observed results:
You and the horse will glitch and repeatedly take fall damage.

Expected results:
The horse should just be teleported up a little bit to stand on the dirt without causing glitch behavior.

Video:
0414.mp4
0414(1).mp4