Stewart,
I want to first be clear that I am new to the watch hobby. I only want to pose a possibility based on something else I have noticed. I had never thought of it before because most of the crystals I have seen have been more scratched than cloudy and none has had this patterned "burn". In other words, I don't really have a clue.

It seems plausible that the culprit could be "out-gassing". Many resins, paints and composites out-gas or basically degrade into the atmosphere. Sometimes this is triggered by temperature or even UV radiation (el Sol). You can often see example of this between panes and near the edges of earlier generation of double-pane windows. The gaskets or paint are out-gassing and leaving that clouding around the edges. It's also in your car, notice the color on a white rag after you wipe the inside of your windscreen. I used to think it was cigarette smoke, back in the day, but I still have stuff on the windows.

So, the edges fogging makes me think the gaskets cause it and it adheres to a nearby surface. And that localized clouding could simply be residue, there's not much turbulence in a watch to disperse the vapor. Maybe on a Striking 10th there is.