I think that the suppressor adds quite significantly to the rate and amount of carbon build up. No doubt it will happen w/out suppressor too, just takes much longer.
I have now observed this exact same phenomenon in 3 different rifles - 2x 7mmRM's and a 6.506 Ackley.
I hear ya. My 7mm mag un-suppressed along with others i know unsuppressed was about 350 rounds. Then a good scrub brought them back.
Personally not a fan of suppressors.