OK, so I found the output volume adjust (shift + attack knob). I guess I should RTFM :) That does leave the impact of the 'decay / sustain' knob on my volume. Same goes for the different oscillator modes by the way. Mode 8, 9 & 10 (the organs). They feel less strong than the other modes. But please correct me if you have other experiences...

Yes the newest firmware seems to have fixed this issue. It now stays in sync. Thanks!