Random structures generating in Superflat world with the Overworld preset

So I was just creating a new Superflat world with the Overworld preset (without changing anything) and I stumbled upon some random structures (mostly ocean related), that shouldn't belong here. Don't know if it WAI, but it seems like a bug to me.

(EDIT) I just noticed that when I use the /locate command it also doesn't work on finding these structures