You can’t activate PiP mode during app enters background mode.
See docs -> https://developer.apple.com/documentation/avkit/avpictureinpicturecontroller
Picture in Picture (PiP) is a user feature that Apple intends to always be under user control. Invoke PiP only in response to a user’s explicit request to do so. If an app invokes PiP in a way that isn’t under the immediate direction of the user, the App Store review team will reject it.
So try to use a UI button to activate PiP mode.
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