I don't really know what happened, and no other leash bug seems to be this bug.

I decided to spawn an iron golem using /summon (I put

to try to make it invincible. I used an NBT editor to make it invulnerable).

I then tried to tie it to a nether brick fence (which already had snow golem attached to it, which have their own issues) and the lead got attached to the block ABOVE the fence. I don't know how this happened...