If you leash a mob to a fence while not holding a lead item then you won't be able to break the leash knot.
If the fence is broken, the leash knot texture hovers in the air indefinitely, and the lead breaks a few seconds later. Re-entering the world fixes the leash knot.
Related issues
is duplicated by
Attachments
Comments

Works fine for me.

Confirmed !
The fact is that leashknot has an hitbox. But they're unbreakable...

But the thing I can't understand is that sometimes it works but sometimes it doesn't...
Use right click instead of left click.

Confirmed for
16w39c
Sometimes neither right nor left click works

Relates to MC-108225
Happens when you leash a mob to a fence while not holding a leash knot.