MintLP Protractor Issue. Anyone else?

Hello,
I went to use my MintLP Protractor last week to do a re-check of Arm, cause I had dis-mounted the Arm.

Last time I used it, was when I first recieved it, over a good year ago, maybe closer to two years.

What I have noticed with mine, that it was once a frosted transluscent clear overlay over a sheet of glass mirror has now changed.

Now I notice, that clear frosted color of the overlay has turned a dirty brown stained color. Anyone else have this happen?

Whether any other detrimental effects have occured, I am unsure, but this Protractor was always kept wrapped in its protective Plastic Bag, in its Cardboard Shipping Box, it shouldn't have turned a brown color, expecially not for what this protractor cost.

I'm thinking of emailing Yip, and asking why this happened, and what can be done about it? Mark