That is a wear crease in the leather that was present before the damage occurred.
If that is so, it is not a concern…only that the damaged top coat need restored after the stain removal.
I'll have to read on your site about fatliquoring.
Hydrating and fatliquoring is highly recommended as the leather structure has been penetrated with foreign contamination of sort. Hydrating helps to wicks up these contamination and restore its pH balance charging the protein leather fiber positive for more effective hydrogen-bonding with the negative fatliquor.
I also tried some APC with a medium brush with no success.
Most APC are alkaline you may want to neutralize it with a leather acidifier to return to the leather chemistry integrity of pH 3 – 5.
I am kicking myself for not getting black leather.
Are you going back to the era of Henry Ford? Take a look at the below pictures…does the first picture stain looks worst than yours…and the result of leather-safe cleaning?
Once the non-leather-safe residue is neutralized, Stain removed non-destructively, re-topcoat it…it will return back to your original appearance.
Now to keep its appearance high at all times, a routine non-stick rub resistant protection will keep you worry free!
#2 Blue Jean Dye Stain
Roger Koh
info@leatherdoctor.com