I am having the same issue. Except it doesn’t play if the screen automatically turns off or if I press the side button to turn off the phone. It just stops playing.
Are you doing this in the preview? Mine also stops, but if I build it out in Test Flight, the background audio works and even shows on the home screen.
Actually I’m using the PWA (which I intend to use for my clients as well). If I lock the iphone (right button), everything works as expected, but once I try to swicth to another app, the audio stops.
Maybe I do something wrong but I don’t know what for the moment