Steps to reproduce:

1. In creative, disable mob spawning.
2. Go into a cave and place a mob spawner where it's dark enough to activate.
3. Place any monster spawn egg (creeper, skeleton, zombie, spider, witch, enderman, endermite, zombie villager, and cave spider all work) in the spawner.

Expected results:
The gamerule being disabled prevents the spawner from spawning mobs.

Actual results:
Mobs spawn despite disabling the gamerule.