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.
You can add your voice and vote to and existing feature request if you’d like.