The bug

Boats seem to have janky motion when hanging from a lead. This is most notably observed by if you leash a boat then pillar up, the boat will start flying way up and even if it doesn't go that high eventually it will start flying way up. this doesn't happen to nearly this degree with other mobs. I suspect this is caused by the "floatiness" that boats have, so their upward momentum from the elasticity of the lead isn't countered by the downward momentum from gravity as harshly. However, chickens have similar floatiness and don't experience this extreme elastic effect

Steps to reproduce

1. Spawn 3 mobs
2. Put 2 of them in the boat
3. Put the third one on a lead
4. Put the boat with the mobs on a lead
5. Fly around slowly and change directions a lot
6. Observe the boat will fly way up in the air   (the mob sometimes does too but it is a lot less frequent)
7. Stand on a pillar that's high enough off the ground that they are hanging
8. Notice the boat will start flying way up and down without really losing any momentum (if any gaining it), while the mob not in a boat eventually starts oscillating in a 2-3 block range

The oscillation of the boat is sometimes difficult to reproduce so it might take a few times of testing. nothing like this happens with other mobs like pigs or chickens.

Expected result

Boats on leads would have normal physics and not gain weird upward momentum

Observed result

Boats on leads have bugged physics and gain weird upward momentum sometimes

Video example:

https://www.youtube.com/watch?v=dwgIP7tpg3w