This problem with laser-etched PBT keycaps has come up & been discussed in the past, especially when the Vortex Poker3 was introduced. Here's what happens:
The surface of the PBT keycaps is pretty smooth. When the markings are burned in by laser the area of marking is, on a microscopic scale, rough. As you type the rough area of marking will pick up bits of skin, oil, & dirt, obscuring the markings. What you're seeing is not the markings fading, they're just getting dirty. Cleaning the keycaps will bring the markings back to their full glory. The color of the markings is permanent, BTW.
I know this probably doesn't help 'cause most folks don't want to pull their keycaps every few months to clean them. Painting them with clear nail polish to give the caps a uniform smooth surface, maybe?
Dyesub PBT caps don't have this problem, doubleshot PBT caps don't have this problem, only the PBT caps marked by laser.