it is being exposed to light, and all light is bad! But to varying degrees. Using a UV sheet will still result in fade, over time. That time will be longer if the display is not in direct sunlight, and longer still, if not displayed in direct lighting. Incandescent light is worse than fluorescent. I do not know where LED lighting comes in, but I'm sure it is bad.

Clay-composition chips with paper inlays are in more danger than ceramic chips.

For what it's worth, Michael's sells UV glass in different sizes.