My gut sense is that may not be it, but it would depend on whether light can get far enough past there to bounce around inside. You could put some black tape over that section to test out the theory. That side section is part of the insert. I looked at one of my backs that is handy. On mine there is a rib horizontal across the interior of the insert, between the film rolls, that provides a gusset which potentially makes that side plate rather rigid.

That leaves me to wonder if maybe there could be some sort of structural damage to your insert that is the root of your problem. Are there any cracks in that internal portion?

But then, the side section appears to be a thin outer cover over the inner injection molded part, so I suppose the outer cover might be bowed away from the inner part with an air space between. That may or may not be a problem, hence the encouragement to use tape and test.

The closure around that portion of the back does not use foam seals, just a sort of labyrinth arrangement where a rib on the insert goes into a channel along the back shell which doesn't seem to be very tight, suggesting it's not critical.

(This is another area where some spare modules to interchange could be handy for diagnosis.)