When I was exploring 116 Pre-1, I created a super flat world using the "Redstone Ready" world type, when I summoned the Wither(I do not think this is the cause but I may help) far out from the spawn, when world loaded in it generated as a regular world, mobs and everything. I have attached screenshots so you can see what I mean. I was about (X=10, Z=230) when I saw this