First my screen goes off.After 15 minutes the app widget dissapears, music stops, and i have to reopen app. So i have done everything to fix this. even the trouble shooting guide. this is my phone and what i use.
-zte grand x4 model z956
-android 6.0.1 marshmellow
yes i have the battery optimization off for spotify, i have tested it using wifi and celluar data both end up the same.
It sounds like something on your mobile is killing the spotify process when it is running in the background. Does it happen with other apps too? The usual culprits are apps like cleanmaster and clean sweep or any other memory/battery managers. Disabling such apps or excluding spotify from them should fix things.
i have done a complete wipe on my phone and deleted any apps that are not required on the phone. i have tested it with battery optimization on and off, also have not downloaded any other apps. plus i just tested it on my older zte sonata 3 and it works.