Hi, i have an 04 roadster and have noticed this crease appearing.
I lowered the top to see if there was any obvious reason for the marking but have come to the conclusion it is simply the way the material creases/folds as the roof lowers.
Maybe going to try some tape across the area to make it crease across the roof instead of front to back.
Also going to refurb the fabric with waterproofing/conditioner, this may make the fabric crease in a better way.