Mobs teleported to different dimensions while leashed causes ghost lead/ lead duplication

The bug

Reappearance of MC-15270.

If a leashed mob has somehow walked into a porta, the lead item drops but the mob is still leashed, causing a ghost leash to exist on leashed mobs. This can easily be done in creative.

This can also cause leash duplication if the player puts the leash to a fence and breaking the fence, leaving the leash entity floating until it breaks into a item.

How to reproduce

1. Spawn and leash a mob
2. Drag the leashed mob into a portal
   → The lead drops but the mob is somehow still leashed even though it just went through a portal.
3. Leash the mob to a fence
4. Break the fence but don't hit the leash entity
   → The leash remains floating until, it drops as an item.