White debris on front of cartridge

Schmutz won't foul the operation of an optical cartridge?

Seriously (as opposed to my previous post), the difference you are seeing between the two cartridges accumulating white schmutz is probably only due to some difference or differences in the shape of the cartridge bodies such that one is more prone to accumulate debris where it is immediately visible. If you remove the cartridge that appears "clean" from its headshell and examine it in areas normally hidden from view when in use and under a low power microscope or even grossly, I would wager you will see white stuff there too. I have seen fine white particles on cantilevers mostly, using a microscope, but otherwise I have no idea where the residue might be coming from.

The technical term for that stuff is “Schmutz “.