I tested it on iPhone and it was never called.
But on iPad, I managed to get "scene is now in the background!" printed on the first launch.
I thought the print statement might not have worked in the background/while inactive, so I tried saving the scene phase to UserDefaults inside .onChange. It also didn't work.
But according to Apple, .onChange should be working. And it probably is, but I couldn't figure out a way to test it.
I'll let you know if I get any new developments. Thanks for the comment!