When moving a leashable mob through a Nether portal with a lead, the leash can disappear completely if the mob and the player don't use the same portal.

To reproduce

1. Place two nether portals, about 10 blocks away from each other in the Nether, and connect them to Overworld portals
2. Summon a leashable mob
3. Leash it
4. Move it through the first portal, but don't go through that yourself (you need to be in Survival mode)
5. Go through the other portal
   → The lead disappeared without any trace