From your description, that seems to be the background photoreal textures which, incidentally, is one of the few things that we do not control with our software, so it's an internal optimization the sim does on its own (depending how much free memory and video memory you have), which can happen with any scenery.
Maybe you don't see this in other FSDT sceneries, because in later sceneries we did more and more custom memory management, for the precise reason to not let the sim control things too much, which usually results in issues like this.