Send timed metadata to controller

I have a custom player built on top of SimpleBasePlayer. From this player, I want to send frequent time-accurate metadata to attached controllers (especially the UI controller). It seems like setTimedMetadata would be an appropriate solution. So within the simple base player I update and invalidate the state as follows.

private var state = State.Builder().......build()

private fun sendTimedMetadata(metadataEntry: Metadata.Entry) {
     val time = 1000L
     state = state.buildUpon()
            .setTimedMetadata(Metadata(time, metadataEntry))
            .build()
      invalidateState()
}

where sendTimedMetadata is called, when I want to share the metadata.

When I attach an Player.Listener to the directly to the player, this it receives the metadata on the onMetadata-callback as expected.

However, when I use the regular setup with MediaSession and MediaSessionService and trying to connect a MediaController from an activity, the onMetadata is not called for the controller. Other state changes like isPlaying are working fine.

Am I missing something here or is this not expected to work? Maybe there are alternatives or better ways to send regular events (could be 10 events per second) to an activity? There seems to be broadcastCustomCommand of a MediaSession but currently I did not find a way to listen to such commands by controllers.