Yes, it's normal. Any suppressed autoloading firearm is gonna be far dirtier than unsuppressed. And the gas system in the AR hasn't a thing to do with it; all the crap comes back through the bore. The orange followers in my S&W 4516 mags are grey-black after just a few mags.
Advantage to the AR: The Stoner gas system, which is not direct impingement, provides for less port noise than any short or long stroke op rod system. It is one of the quiestest semi-autos suppressed. Many rifles aren't hearing safe regardless of suppressor due to the port noise.
Action type & Effects on SPL
Suppressed sound levels referenced to an M4 Carbine
(M855 ball ammunition, 14.5”
barrel, same suppressor)
HK 416 5 dB louder
SIG 551 14 dB louder
Steyr AUG 9 dB louder
Bolt action Same (however, barrel was 20”)
AR-15 2 dB quieter (20” barrel)
I believe this is measured at the shooters ear, rather than 3' to the side of the muzzle.