I am 0-for-1 with museum glass over a marker piece.
Had it framed on the wall for 3-4 years. Not direct sunlight, but there was a window in the room (not facing the artwork, curtains mostly closed). One day the person I got it from was visiting and saw it on my wall.
"WTF happened to the Miller??" he asked.
"What do you mean? It's framed on the wall right there," I replied. "I love it, it's great."
He assured me it was not "great" and insisted we take down the piece.
We removed it from its frame, and sure enough, my friend was right. The bottom type which reads "BFK#2" had been covered by the mat while hanging. The rest of drawing used to be that dark. I didn't notice it when it was fading one day at a time.
P.S. yes, I had the glass installed correctly...