I've noticed this same problem and I have the same setup (Nexus 6P, Android 7.0, etc.). I believe this is actually a problem with the phone (6P) and Nougat (Android 7.0). I've tried music playback in several other apps and they all do the same thing. I've also tried playback locally, streaming, through Bluetooth, headphones, and internal speakers. All the same issue and I've confirmed that the issue seems to exist in all music apps (for me anyways) regardless of if I'm playing through Plex, Google Play, or Amazon Music. SO far I've tried 1) factory reset, still has pausing issue with no additional apps installed 2) removed battery optimization on Plex, same issue 3) disabling "OK Google" constant listening, still same issue 4) disabling ambient display, still same issue. You might give one of the 4 a try (especially disabling OK google) as they seem to be working for some 6P users.
I've contacted Huawei, but they refuse to respond up to this point (sent 5 e-mail and called twice). My guess is that this is a Nougat Bug at this point and hopefully the release of Android 7.1 in December for us will fix the issue. Until then it seems we have no hope.
I'll post if I ever find a solution, but have somewhat given up at this point. Please let us know if you find a solution as well as I'm excited to be able to listen to music on my phone again :)