The Play/Pause and media control buttons by default, launch and control the OS native Music app. The default functions of keyboard media control buttons are defined and set by  the OS itself and therefore cannot be set in Logi Options+.

If any other media player is already launched and running, for example, playing music or a movie onscreen or minimized, pressing the media control buttons will control the launched app and not the Music app.

If you wish your preferred media player to be used with the keyboard media control buttons it must be launched and running.