Spotify Controller goes useless Galaxy S8

[Air2Desk]

Hi all, hope someone can help. 

With battery saver on mid setting, I can start using Spotify with no trouble to connect to a Chromecast Audio or Bluetooth speaker. After the screen shuts off for a while with music playing, if I turn the screen back on, the controller is still there but the controls do not work. Actually, they work but not for what is being played through the connected device. 

Example: I am playing song "A" over the speakers in my house (Chromecast.) My screen goes to saver black. I want to skip this song and so turn the screen back on. The Spotify Controller is there and shows the song info, etc. I push skip. A new song, song "B", starts playing, but over the phone's speaker WHILE song "A" still plays through Chromecast. There is now no way to control the Chromecast music. Crazy. 

In this instance, the only way to get Chromecast control back is to click the app tab that shows all open apps, and close out Spotify. Once I re open Spotify, it asks me if I want to continue with connected device or use phone's speaker and all is back to normal (after selecting continue with connected device) until the screen goes to sleep. Maddening. 

Alternatively, I can fix this by turning off battery saver, which is not what I want to do. I have to do this for using GPS too. 

Note: I do have Spotify selected as an app to allow in background in the battery saver settings, but this has no effect. 

Anyone know what is going on or what I can do to be able to keep battery saver on mid setting?

Thanks everyone!

DM