I don't think you will like my answer, but it looks like you scrubbed the toe of the left boot (right side in the picture) too hard and scuffed the leather surface. The scuff opens the leather grain and allow the oils from the Obenaufs to be absorbed more readily in that spot than other areas of the boot. I really don't know of a way to fix the problem, but perhaps someone else has a better idea (or a different opinion).
Ugh...that's sorta my fear. I didn't have to scrub hard at all with the saddle soap to get the scuff off, so I'm surprised that that's the case.
I wonder if I can conceal this issue with any sort of polish or other product. Anyone? I *love* the color of these boots, and they've been discontinued for a couple years.