I'd suspect a bellows pinhole toward the front of the camera or light leak around the lensboard that is small enough that it doesn't have much effect at shorter exposure times. It would cause overall fogging of the negative at longer times, but not the part of the film rebate covered by the filmholder rails.
I had a similar problem on a camera with bellows that appeared practically brand new. There were a couple of corners that had small holes. Fixed now
Do the flashlight/lightbulb test of your bellows and lensboards to rule this out before looking elsewhere.