I believe that a couple of years ago, we lost the option to turn off the TV display while listening to Spotify. Occasionally, I would try to use Roku's phone app, giving a voice command to achieve this, as it worked in the past. However, it no longer seems to work today.
After some tinkering, I discovered a workaround. Here's what you can try:
1. Open Spotify
2. Play a song
3. Choose AirPlay to Main TV
4. Close your phone's screen
5. Reopen your phone's screen
6. Go back into Spotify
7. Instead of using AirPlay, select the network's Main TV
Note: There's no need to turn on the TV during any of this.
If you're like me and prefer just LISTENING to music (no dimmed screens with titles, no static artist graphics), give this a shot. It worked for me, and I hope it works for you too!