With an empty community folder, I had skidmarks and cracks textures on the runways at KTVC (they look really nice). After adding GSX, the skidmarks were gone. It looked just like a FS2020 runway. After removing GSX, the skidmark textures returned. I then added the Fox Two Models KTVC, and lost the skidmarks again. After removing the 3rd party scenery, the skidmarks returned.In my situation, I think GSX removes the skidmark textures from most airports because it interacts with most airports while individual scenery just removes them from that particular aiport. Just a guess.
I noticed that too recently. I would suggest reaching out to FSDT support!