i don't think the dampers just simply allow the ink the "fall out" of the printhead. the system is open/closed via the capping station & ink pump operations, refreshing the siphon-effect of the ink supply, but stops/closes the flow when parked on the capping station with "closed" inkpump lines beneath it.
my guess would be you have a bad ink pump that's either not closing, or has a hole/crack in the ink line, that allows the ink to continue siphoning out through the printhead/cappingstation. if its not in your ink waste tank, all that missing cyan might be found in the tray that sits below the ink pumps inside of your printer (accessible via a back cover panel on the cappingstation-side of the printer)