I was also having this problem with some Bluetooth headphones.
I usually used to use headphones with Bluetooth version 2 which worked with Poweramp v3.
When I switched to these new headphones which run on Bluetooth version 4.0. Poweramp v3 did not respond to buttons apart from volume (but I suspect that was interacting with the android OS rather than PA). I first thought it was the headphones but seeing it be okay with other apps i realised it was the app. This topic made me consider downgrading from Poweramp beta V3 to Poweramp V2. Headphones now work fine.
Hopefully that long