I looked at the valve and it was clean. It slightly changed its color but nothing other than that. No signs of debris or deformation as far as I can tell. I put it all back together and unsurprisingly the problem is still there. Maybe the problem is on the joystick side where the spring is not opening the valve enough when heated

Did you end up finding a solution to this issue? We're having the same issue with both steam and water valve on a sync.