In my pack I have somes mobs added on spawner.json question to have more spawning in world. In my creative flat world I made for the dev of my modpack, I set the vanilla gamerule "doMobSpawning" to false, question to stop all the annoying mob spawning when is not needed, and after killing all mobs, the mobs added on spawner.json file start spawning back.

Spawner.json config:
https://pastebin.com/zajhw8gx

InControl: 5.1.9
FxControl: 2.0.11
Minecraft / Forge: 1.16.5 - 36.1.32

i'm having this same issue