I am having almost the same exact issue with my Rolleicord Va. Always the 12th frame. But the ghosting appears more in the center of the frame than on the edges. This has happened at labs and when I develop my own. Always the same spot. Used to happen infrequently, now it happens on every roll.
If it's happening now on every roll then do this test.........................load the roll and remove the roll in total darkness and see if the light-leak ends. I'd be willing to bet it will be gone. JohnW