I had the same problem with beta 795, Xperia XZ2 Premium and Sony WH-1000XM2. Switching from hi-res output to OpenSL ES solved the problem, but switching back to hi-res did not break it again (and I'm pretty sure it switched back properly, cause there's noticeable difference in volume between the two). Now I can't break it back even if I restart Poweramp or reboot the phone itself. Weird, but hopefully helps someone.