When doMobSpawning is set to false, entities will still spawn in newly generated chunks. It's as if the chunk already has the entity data in it, then once the player loads the chunk, the game processes the entity. This only goes for all passive mobs, however, the game wont spawn more entities since doMobSpawning is disabled, just generated chunks will.
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However, this issue is a Duplicate of MC-118347.
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