So I cracked open my mags for a good maintenance session, their first.
The one that came with my Socom Gear M9 was leaking, and the o-rings needed an oil bath.
The spare, (forget brand) which wasn't leaking, when I opened up saw that it has some cardboard packed, neatly I may add, in the magazine hollow where the gas goes.

Is that supposed to be there? I imagine all it's doing it limiting the amount of gas I can fill it up with.

Strange things like that are not unheard of depending on what brand you purchase among other factors. Odds are the Cardboard was part of filler material used to separate the mags during transfer from one part of production to the other. They use it to space mags to prevent marring of the surface of the magazines to make them look like they are in pristine condition until the point you purchase them. Odds are when someone was adding the valves at a different stage, they did not check the mag for remnants of the packing material. Such is a problem with supply chain management, you must balance quantity, quality, and price in a manner that still allows your business to profit. The logic is the more time spent on quality control and redundancies, the more expensive the end product will be.