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 drawn out explanation will help with SEO.