Hi Chris,
I've noticed the same thing with my Paasche H. My normal cleaning regimen with it is to spray a color cup or two worth through it, intermittently back flushing it. I only occasionally do a full a full breakdown. If there is any stubborn residue, usually Acryl Flat Black, I use lacquer thinner to clean it off. I don't worry about too much. I've never had any residue actually affect the air brush performance.
I follow the the same sort of regimen with my Badger 200 NH (single action 155), but since I use only enamels in it, it's rare that the occasional breakdown cleaning reveals any residue.
Andy