I can confirm the bug (Samsung KU6079UXZG - 2016 model, optical Output): when I switch from optical out to tv speakers, I can hear (and not only see) Spotify playing. Sometimes it helps switching from optical out to TV Speakers and back to have Spotify play, sometimes I have to start eg the TV program (external Kathrein-Box as receiver). N.B.: I keep the TV speakers off by having the volume set to "0"; sound comes through the optical connection to my Denon 5.1 anyway.
Encountered the same issue on my Samsung UE55JS8000.
what worked for me:
1. Log onto Spotify app on the TV directly; not using the phone or tablet.
2. Open Spotify on tablet/phone, and select the TV from the devices list.
3. Control the Spotify app from the tablet/phone, and the TV-speakers and/or optical out to the receiver work fine.
The TV-speakers and optical out also work fine, when only executing step 1.
A really simple fix I found, is to go to 'Sound' In the menu, and select 'additional settings'. Changing the audio format from dts neo to Dolby Digital worked amazingly. In my mind the clarity is even better!