Hostile mobs will either despawn or not spawn at all when spectator mode is entered.

I entered spectator mode to try out the different shader tints on Endermen, Creepers, and Spiders by spawning a lot of creepers.

As soon as I entered spectator mode, all the creepers despawned immediately. No hostile mobs spawned at night either.

1. Spawn hostile mobs or wait until nighttime.
2. Enter Spectator mode
3. Hostile mobs will despawn or not spawn