On my phantom 3 Pro, I am able to hold down the rear right button and roll the camera adjustment wheel to adjust the gimbal roll.

When I do this on the Mavic pro by holding the c2 button and turning the camera adjustment wheel, it also simultaneously changes the camera EV as if I weren't holding down c2. Is this a known issue or am I doing something wrong?

You could change camera EV back after adjusting the gimbal roll.
Thanks for your feedback, we'll get it reported.