When you lead a boat to a fence then make the boat fall in the void the boat will disappear but the fence will still have a lead on it, upon reloading of the world the bug will still be active.

I think the game does not notice that the boat is no longer there also it is not a render problem because I ran the /kill @e command and it did not say boat in the output but it did say leash knot.

PS You can do this in the end while being in survival mode 

PSS If You Left click the lead thing it will take it off the fence.

PSSS video is in 1.16 but same in 1.17