Hey, Justin! The RB cameras are great, just got one myself about a month ago, what a tank!
Did you take any shots with the back rotated to landscape? Not that I’m overly familiar with this system, but I would think that if you are getting the same or similar leaks with the back in both landscape and portrait, then suspect the seals on the back itself, maybe the seals on the rotating adapter. You can also check for light leaks in the bellows. Camera on a tripod, remove the lens, and use a small, bright flashlight. Take it into a dark room, and with the viewfinder closed and the room lights off, get your eye close to the lens mount, looking into the bellows, while you move the flashlight held close to the bellows. If there are any leaks, they should stand out plainly.
On the 6x4.5 Pro back I have, I am getting a consistent leak along the right third of the negative with the back in portrait. In my case, I didn’t take pics in landscape of subjects that would show the same leak, so the next roll will be of a suitable subject specifically to determine any leaks. As I just signed up for this forum today, I have not searched for other threads in regards to this issue, however, I’m sure there is a wealth of information from members on this topic.
I will say, Fix Old Cameras on YouTube has concise videos on replacement of light seals, also an excellent video in the repair of bellows on a Mamiya 6 or 7, I believe.
Have fun with your camera, it’s a great system! Keep shooting!