Pillager-Outposts doesn't spawn in Superflat-Worlds

I don't think that's a bug. The reason villages spawn at all is because in the preset it says 'village', and if you put pillager outpost in the superflat generation preset there would be too many of them and the world would be covered with pillagers.

@a Thats not how spawning works for structures or mobs (want proof, load up a surface plains buffet world)

Not fixed in 19w14a unless you use pillager_outpost tag in box.

Yeah, that's the purpose of this tag... So flat worlds can be generated with only the options that are needed.