Look at this: Chicken Invaders Universe/ Background Bug - YouTube
This feels weird, when I was in the game the CIU Background 2 was used, when I left then rejoined it used CIU Background 3 (Which is normal) but then, after the red background was gone (from completing the stage) the CI2 Background was there, not CIU Background 3. This might be a bug. (Also if you think this is used with random backgrounds then no.) (Literally just copy pasted the description of the video lol)
@InterAction_studios