Android media buttons not working while activity is not visible

Question

I'm having trouble getting the external media buttons to work in my app while the activity is not visible.

The media buttons work correctly while the activity is visible. I can play and pause the player without any issues by pressing the media buttons (while the activity is visible).

As soon as I minimize the activity (by pressing the Home button), the app stops responding to the media buttons.

I know this question has been asked a couple of times but it seems that I'm missing something that the other questions don't cover.

Here's a minimal reproducible example: https://github.com/HectorRicardo/android-media-buttons-not-working-mre/commit/a4ea01bf29ba7289e055767c5c935e35b7fbbcff

(the left side of the diff is basically a blank project from scratch created in Android Studio, while the right contains the minimal reproducible example)

What I have done so far:

• I've made sure that the PlaybackState always contains the ACTION_PLAY, ACTION_PAUSE, and ACTION_PLAY_PAUSE actions, for example:

mediaSession.setPlaybackState(playbackStateBuilder
        .setState(PlaybackStateCompat.STATE_PLAYING, 0, 1)
        .setActions(PlaybackStateCompat.ACTION_PAUSE | PlaybackStateCompat.ACTION_PLAY_PAUSE)
        .build())

• I've added a MediaButtonReceiver to my manifest:

<receiver android:name="androidx.media.session.MediaButtonReceiver" android:exported="true">
  <intent-filter>
    <action android:name="android.intent.action.MEDIA_BUTTON" />
  </intent-filter>
</receiver>

• I've updated the onStartCommand of my MediaBrowserService:

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
  MediaButtonReceiver.handleIntent(mediaSession, intent);
  return super.onStartCommand(intent, flags, startId);
}

• I've made sure that the media session is active. Inside the onPlay method of the MediaSessionCompat.Callback, I do: mediaSession.setActive().

• The FLAG_HANDLES_MEDIA_BUTTONS flag seems to be deprecated according to the reference docs. I tried adding it anyways, but it didn't help.

Not sure what else to check for. What am I missing?