Mabye caused by the fix of
MC-21433 you are unable to leash multiple mobs to the same fence if you are not holding a lead item. This happened first in 16w39c.
You can leash like it was in previous versions as much mobs as you want to a fence regardless of the held item.
- Place a fence
- Leash one mob to the fence
- Try to leash another mob to the fence while not holding a lead item
→ It does not work
- Try again while holding a lead item
→ It works