Well, a voice from the other side. I just checked fabrics in my stash, on open shelves, in my utility room. One window with no blinds or curtain, however, there is a carport on that side of the house. I pulled a piece of fabric that I know I have had for at least 6 years and I don't know how long my Mom had it.
There is no fading along the fold line.
I don't have an explanation for this and just don't worry about it.