Mob cap does not work on realms

Mobs spawn after hitting the mob cap (monster cap) on my Realm. I built a room with at least 140 zombies to avoid spawning at the surface but hostile mobs still spawn. They spawn even when I am on the server alone (only active chunks are in my distance) and I am at the bottom of it. 
I tried the same thing on Sg world and Hostile mobs stopped spawning when i reached 70 zombies in my room.