The Bug

Attaching a boat that is on a lead to a fence that already has another entity leashed to it removes the lead for the entity already on the fence, rather than attaching the boat to the fence. This is unlike any other entity.

Also, as Bansed stated, this bug only occurs when right-clicking on the leash knot part on the fence; if the lead is used on the fence instead, the boat can be leashed even when there is another entity attached to the fence. However, other entities can still be leashed onto a fence by right-clicking on the leash knot.

How to Reproduce

See this video: MC-272789.mp4

1. Place a fence and two boats near it. You can use any other entity that can be leashed (e.g. cow), but the boat must be the last entity to be leashed onto the fence.
2. Attach one entity that can be leashed (e.g. boat, cow) to the fence using a lead. Make sure the leash is not attached to you!
3. Attach the other boat to the fence using a lead.
4. Note how the leash knot for the other boat is removed rather than adding the other boat's leash to the fence.
5. Trying this with other entities (e.g. cows) produces the expected result, where both entities can be attached without having to remove one.