Zombie Pigmen, when attacked, will swarm the player as intended. However, on the Realm on which me and a number of friends play on, the Pigmen do not de-aggro, no matter the distance between them. Having gained over 300 blocks from the affected Pigmen, new Pigmen are continuing to attack without being called, and already affected Pigmen do not lose their aggro, even after gaining significant distance and avoiding sight for well over 10 real life minutes. This makes navigating the Nether very difficult, as any and all Pigmen now attacks the first player they see from most locations.