Further off the wall speculation -- it might have just now appeared after a number of rolls because a little bit of attachment adhesive worked its way into a bad spot and tore a chunk out of a seal when the back was opened.

The struck area is toward the bottom of the image, hence near the top of the camera, as the image is inverted on the film. My AE-1 is temporarily inaccessible, but I recall some cameras have a little nub sticking up in the light seal channel that resets the film counter when the back is opened and closed. If that is true of the AE-1, there might be something awry there, as there could be a bit of a gap in the seal.