@Cheddar_threads : Hey man, not IH crew, but to try to help you finding what the stains might come from, I must say I'm not sure it's from the leather.
I did the same as you by forgetting the strap for my first wash, but basically the only result was that the leather strap was completely dry. Would be strange that leather would do this since all jeans patch are leather.
Not sure how you got this though...but I would first try to re-wash it simply if not done already