Steps to Reproduce:
Place a boat or raft with chest.
Put a mob into the boat, such as a chicken.
Attempt to use a lead on the boat.
Compare the results with using a lead on a boat with chest without a passenger in it.
Observed Results:
When having a passenger, the boat/raft with chest will open its inventory. When having no passenger, the boat/raft with chest will be leashed.
Expected Results:
Regardless of having a passenger, the boat/raft with chest should be leashed.
Other Notes:
Using a lead on the boat while sneaking will not work either, it will still open the inventory of the boat.
If you attach a lead to the boat before a passenger enters the boat, then the leash will remain. However, you can no longer remove the lead by pressing use on the boat.
Video example: https://youtu.be/BTYL1J-zINM
Can confirm. It's also not possible to unleash the boat once you leashed it and a mob rides the boat as you can see in the attachment.