I have had a similar leak. In my case the source was at the leading edge of the camera back where it mates to the bottom of the camera body when closed.
On some Rolleiflex models you will find black yarn in the channel/groove at the bottom edge of the camera back; others not. If yours does not have such a light baffle, I suggest you try installing one. Use either black yarn or a thin strip of black "foamie" material.
Here is what happens and how it matches the phenomena you describe. A tiny leak enters at the bottom/leading edge of the back. Through most of the roll it is not an apparent problem. That is because the leak is blocked by the paper backing on the film. It cannot take any path to the unprotected emulsion, and so is not detected.
But as you wind through more exposures the circumference on the supply spool diminishes. Starting on about frame 9 or 10, the supply film circumference no longer blocks the light entering at the base of the camera. The path of light may then take a good bounce off the bright shiny film spool pressure/guide which is no longer in contact with the film. The bounced light then strikes the emulsion in the area around the rollers ahead of the film gate. This is a pre-exposure. The next time you wind the film, the exposed area is within the film gate. Thus you will see the leak in about the same position and shape on each of the remaining frames, and usually getting a little more pronounced with each frame.
It took me a fair amount of time to figure this out, because the problem seemed intermittent. Here is an example frame 11 from a 3.5E before I fixed it:
In this case the flare is almost fortuitous, as it appears to be related to the high voltage transformers. But most of the time it is ruinous.
Fortunately the fix is easy. I took the "foamie" route, the stuff with peel and stick adhesive. Purists will prefer the black cotton yarn and a dab of glue.
Hope this helps!