I have been experiencing this issue since su5 and the most recent update did not change it. Earlier I reported that the sim crashes when the custom CGL scenery is in the Community folder and the location is selected in the world viewer, that problem can be avoided by deleting the rolling cache.
The darker peninsula that includes static waves is part of default and I know from experience that the transition between custom and default represents the border between two QMID grid cells. Of course I'd gathered imagery from that sector, which requires downloading the entire sector, but only rendering the small areas of the island that fall within it.
I am using the same image source that I used before su5, when everything worked as expected.
Is there any likelihood Microsoft will address this issue? It is reasonable to assume there are other areas of the world, as yet undiscovered, with similar, or other anomalies.