Hi. My answer probably isn't the best for your problem, nonetheless this is the closest solution I can give you. To play your spotify on mobile while still Dex on, go to Media in Quick Setting(Status Bar), change the output audio to your mobile device. The caveat is all your sound including one from Dex session will be played in your phone speaker. Spotify actually allow us to control which device to play our song, however since the Dex and phone system run on the same device your Samsung phone, it couldn't be done in this case. So, just follow the Media -> Output Audio I gave you. Cheers 🥂.
