Spotify seems to be a real battery hog, but are you in a low-signal area? (especially on LTE). I've noticed a big difference in battery life on the S8+ (and to some extent, the Note 9) based on whether I'm in a "1 bar" area or "4 bar" area.
* Spotify is inefficient (wonder how much is DRM?)
* Low signal causes your phone to increase transmit power.