2 day old S7 has same problem. One fix I have discovered is: Go to Pandora settings, Advanced, Screen Auto-lock. Where is says "conserve battery" select "off". It will tell you that more battery will be used but it keeps the screen on when Pandora is streaming. I use it in my car so battery use is no problem. Hope that helps till Android does
I was having the worst time trying to figure out what was wrong and it was driving me crazy. I have an S6 and this month downloaded the Marshmallow update. Never had problems before with an update until now. Pandora and iheartradio wouldn't play or would stop. Here's what I found that worked for me (took about a week to finally get it right, hope it stays right!) and I hope it helps you..
1. In wi-fi settings, turn setting to "always" for "keep WiFi on during sleep".
2. Turn power saving mode off or have it turn on only when your battery is low.. you can set the percentage.
I will probably turn power saving mode back on in the morning to 20% or 50%. But tonight, my phone was near dead and on the charger and would not turn Pandora on until I turned off the power saving mode. Also, I force stop or close the app and restart it after any change I make to the settings.