Sorry, I haven't thought about this issue for a few days. Let me see what happens if I create a playlist on my Pixel 2 XL to see if it syncs correctly to my other devices.
Also, just to be clear -- you did confirm that the music on those playlists is in your cloud library, and that you didn't specifically select mp3 files that were saved locally on the phone, right?
EDIT: Ok, it works for me. I created a test playlist on my Pixel 2 XL, and it immediately showed up on my Pixel C. (I don't currently have access to my PC or Chromebooks, so I can't confirm yet that it's there on the web version.) Can you tell us if your playlists from the S8 show up on the Google Play Music website on your computer browser?
08-08-2018 07:52 AM