[Fixed] Mobs spawned by Structure Blocks' load feature drop leads

Steps to reproduce:

1. Have Experimental Gameplay enabled;
2. Place a Structure Block on the ground;
3. Spawn any mob inside of the covered area and save the structure;
4. Load it elsewhere;
5. Kill the mob.

Expected results:
The mob drops its regular loot.

Observed results:
The mob drops its loot and a lead as well.