I am in a similar situation:
Galaxy S9+, Oreo
Bluetooth earphone Sennheiser Momentum Free
The volume control works (when vol + and - are pressed both the general bluetooth volume and Power amp volume are changed, also using other apps)
The play/stop button works
The double/triple click options does not work (I have enabled this in bluetooth options in Poweramp)
Observing the last processed commands I see that when I double click button in the bluetooth headphone, apparently no command is received.
But I noticed that a couple of times the skip worked and I think to know what is the problem: the Poweramp timing measurement of the double click is too fast for this headphone model, that probably is a little "slow" to send commands to the phone.
If button is pressed 2 times too quickly, nothing happens
If button is pressed 2 times too slowly, Poweramp stops
Very rarely, pressing 2 times the button, the skip command happens
Solution could be adding in Poweramp a tweak option where the user can set the time interval between clicks to let them understand as double/triple click by Poweramp