Zombie pigman don't spawn from nether portal on easy difficulty

Steps to reproduce :

 - Set difficulty on easy

 - Make a bunch of nether portal in overworld

 - Wait

I tried myself with 15 23x23 portals, nothing is spawning from the portals. I figured it was to small so I downloaded the world of this video : https://www.youtube.com/watch?v=StoefpjrZCo

I waited for 30 minutes on easy but got no spawn at all.