That's the place light leaks almost always seem to appear in bronica backs when the light seals start to go. I.E. this particularly fine example:


I think it's around the hinge seals that the foam is a bit dodgy. In my case, from rather imperfect memory this (and other leaky shots) were the first after leaving the camera sit for a while so the bit of film would have been sitting over the hinge before getting wound up onto the film gate, or it could be around the latch. Anyhow, I resealed the back completely with foam and the problem has disappeared.

This is a good site that has illustrations of the backs and where all the seals are:

I think I got the foam from this guy here:

Though I was getting some camera coverings as well. The chunk of foam I got was half cut into strips and half not, so pretty versatile. I've done the bronica, couple of spotmatics, and an olympus MJU with it so far.