Steps to reproduce:
Create a world with commands enabled
Run /gamerule spawnMonsters false
Run /time set night
Observed results:
Monsters spawn
Expected results:
Monsters do not spawn
Steps to reproduce:
Create a world with commands enabled
Run /gamerule spawnMonsters false
Run /time set night
Observed results:
Monsters spawn
Expected results:
Monsters do not spawn
Unable to reproduce, the game rule works as expected for me.