CustomButton not triggering callback when put first in playbackControl (media hotkey)

I'm working on adding a Media.CustomButton to my media player.
It works fine when added as anything but the first item in the playbackControl list (in PlaybackProfile).

But when added as the first item, it shows up as the media hotkey button, but when pressed the callback to "onCustomButton" is not called.

Am I doing something wrong, or does this simply not work?