Glass panel replacement smoked edges?

First, this part is listed as glass, but it's plastic no? Anyway, I've had several and they all tend to "smoke" around the upper corners over time. Looks like stains emanating from the corners, best visible on a pure white pixel display. Has anyone seen this? I've had 4 now (original + replacement) for 2 machines, and after about 6 months, the situation repeats.