Thank you for getting in touch with your question, and wow - nice DJ rig! What you’re experiencing is normal behavior for grouped volume handling in Roon. We’ve had requests for a 'One True Volume Slider to Rule Them All" You can add your voice and vote to and existing feature request if you’d like.
This post from Mike has a bit more info on the volume controls for grouped zones and why it functions as it does in Roon.