I've only used Kleen-Bore formula 3 Gun Conditioner a couple of times and once used EEZOX. I really wasn't applying these products to the outside of the slide (only the barrel, inside slide, etc.)
I wonder if it was caused by excess temperature during shooting, or weak bonding of the stealth finish at that part of the gun, or something else. It's only a cosmetic concern. Just want to know if has happened to someone else.