I've noticed this same thing. My app is a video/audio calling app, so it shows the video streams and call controls in the app when a user has the app foregrounded. The system buttons are duplicates of this functionality when my app is foregrounded, plus they are dismissible, so I can't rely on their presence when my app is handling a call. This means that when a user answers a call while in my app, they see two sets of call buttons.