I just had a look at my RZ back and there's a good 4mm between film gate and pressure plate; plenty of room to have an offset between flare and image. I can also report that I get offset-flare (on two different RZ bodies) just like you've reported when shooting strongly-backlit scenes*, so it's not a fault with just your body. My guess is that there might be something small and reflective on the back of the mirror and that if that is illuminated during exposure then you get flare.

And it's clearly not a problem with the baffle - the sharpness of the flare image shows that it was formed only while the shutter AND baffle were open, i.e. during exposure. If it was light leakage during handling, the flare would be spread around due to camera motion.

* with the sun in-frame, you don't see the offset because the sun-image is on the film, so you just get lens-flare. This problem occurs with the sun only just out of frame, which causes it to illuminate the inside of the body. My scanner (Nikon 8000) doesn't really go outside the frame boundary because of how the holder works (and I tend to crop out the unevenly-flared edge-bits before saving!), so I can't easily show you an example. But this is IMHO normal behaviour for an RZ/RB.