1. Sneak and right click with a lead on a fence
2. Attach the lead on any other fence
There's currently a minecraft bug (since 1.21.6) where the leash breaks if its stretched over a chunk border and a player dies within the chunk of the leashed entity (which is at the fence post you first connected the leash to)
https://report.bugs.mojang.com/servicedesk/customer/portal/2/MC-300654
