Normally in a default world, pillager spawn as they should at the structure, Pillager Outpost.
When you add the structure into a customizable superflat world, however:
Example of a preset: (minecraft:plains;village,pillager_outpost)
Once you find the structure in a customized superflat world, pillager will not spawn at all!
Screenshots below are shown with proof that I have not been in peaceful mode and in what preset they won't spawn at all.
Presets I used:
Classic Flat
Redstone Ready
Snowy Kingdom
Desert
And all of the other presets!
Duplicate of MC-147360