That is light leaking around the other side of the film rollers. On that back (Not the insert) there are thin rollers at the top and bottom. The bottom roller leaks light sometime when there are internal reflections or lots of sky lighting up the bottom of the film gate, allowing some light to scatter between the film gate and roller to the other side.
View attachment 374133
Light scatters behind this roller.
This happens with Hasselblad film backs too.
If you are seeing this a lot, check for internal light reflections or missing baffles, and/or use a lens hood to limit the amount of light that is not part of the image.