Steps to reproduce:

1. Save a structure with a mob on a lead with a knot on a fence
2. Remove the knot from the fence or
   Remove the fence
3. Load the structure in any other place.

Result: a new knot will always appear on the old place, but a new lead won't appear until reopening the world (possibly because of MC-55851).

• If there is no fence and/or the new structure is far enough, the new lead will break and drop itself.
• If there is the fence, the knot will stay there (even if the lead breaks, but it'll disappear after reloading the world), or else it'll despawn.

The cause is that the coordinates of the knot are saved in the entity, but are not changed when loading the structure

Old description:
I used a Structure Block, saved a house, and loaded it but the lead, attaching a cow and a fence together, broke as soon as it was copied to its new position.