This does happen: iOS writes a snapshot of the screen when an app moves to the background. You can notice that when you open an app that you ran recently and a

This does happen: iOS writes a snapshot of the screen when an app moves to the background. You can notice that when you open an app that you ran recently and a stale, slightly artifacted screen appears briefly before updating. That's the snapshot. But this is at the OS level, an app has no control over it, and it's irrelevant for the whole camera thing.