Mobs still spawn when chunks are generated even if the value of doMobSpawning is set to false.
New chunks spawning Mobs overrides doMobSpawning
"/gamerule doMobSpawning false" does not prevent passive mob spawning
Mobs can spawn in unloaded chunks even if doMobSpawning is set to false
Passive mobs spawn despite /gamerule doMobSpawning false
Wandering Trader and Trader Llama will spawn when game rule doMobSpawning is false