Most apparent with hoglins, but likely affects all mobs.

To reproduce make a lead attached to the mob and then attached to a fence, then despawn the mob. This is easily done with hoglins by traveling a distance away.

The lead will show wrapped around the fence and if right clicked, it will be removed from the fence but does not drop as an item.

This is likely due to the lead being meant to drop near the entity it was attached to but without the entity, the item fails to spawn.