With Xtra 2.42.0 on Android 14, when I start a stream en set it to audio only, and switch out of the application, there's no PiP (which I think is intended as there's no video to show).
But on several occasions where the stream got automatically paused when I take my headset away, once I ask for the stream to play again, it result in a black PiP appearing, and impossible to get rid off without killing Xtra and launching it again. (Stopping completely the stream and starting to play it again doesn't help at all, and trying to remove the PiP stops the stream).
EDIT: Setting is on the default PiP choice for background playback.