No surprise there, the C-6 has the same 1 inch rear port as the C-5 so with the reducer, the effective diameter of the port is about 16mm..
Did you try the 35mm Pan on the C-6 without the reducer?
I should add that Glenn LeDrew has experienced this and has a good explanation..
I tried without the reducer and it was much better. I don’t remember it being bad at all so it had to be minimal. I see no noticeable vignetting with my ES24-68.