Items bounce while resting on leashed boats that are airborn

Steps to reproduce:

1. Leash a boat and either hold it midair or tie it to a fence post that is high enough for the boat to start floating.
2. Throw an item on the boat

Observed result:
The item starts bouncing up and down or phases through the boat after moving erratically for a few seconds (if the boat is bouncing in the air).

Expected result:
The item stays still.

This is different from how items on boats typically work, where they normally just sit on top of a boat without any bouncing or erratic movements. Recording of bug in attachments