When I disconnect from or stop casting on the Android app, Plexamp wipes out the queue and no longer has any indication of the currently active track. The app has no indication of an active track or queue.
The new media notification in Android 11 does show for Plexamp and also indicates the track that was playing when casting ended, but pressing play to restart from the notification does not work.
My use case is when moving rooms, it is great to be able to disconnect from one casting device to connect to another while confusing to play from the same queue. I do that with the Plex app a lot. I also sometimes keep tracks queued up and like to continue playing them over time.