pigmen not seeing you until you see them

What I expected to happen was...:
render distant only effect view

What actually happened was...:
when I set render distant to 2, the pigmen at 8 chunk away didn't trigger until I set render distant to 16 or walk closer

Steps to Reproduce:
1. Set render distant to 2
2. Trigger a pigman
3. Lure them somewhere or kill them

4. make sure you can't see anymore

5. Set render distant to 16

6. you should see pigmen started to see you after you set the render distant