If the mark is in the exact same spot it's not a roller bouncing. Are you sure it's not water from the plate gap?
The Z roller, also known as a bridge, is mainly for when you have light coverage and/or no take off bars. When the roller is disengaged it...
Same watertrain? What shop you at?
I have been on multiple different web heat set presses (G14's, G25's, M1000, M3000) and we've always ran KCMY as well.