With respect to the original supposition, I'd say it unlikely that argon has a direct influence on microwave transmission. It's an inert gas and therefore comprised of single atoms rather than molecules - microwave heating for cooking conveys energy of rotation on water molecules since they have an anisotropy (an electrical dipole) which matches the electric field of the radiation. There is no dipole in a symmetrical atom (unless I guess you apply an external field) and hence no method for absorption. It is much more likely that a coating or the double thickness of glass itself is to blame.
(all those years in the lab weren't wasted...)