Stainless is funny stuff. I have fabricated stainless industrial components for many years.
You can handle two otherwise identical components, be it pipe, fittings, brackets, machinery, tools, knife blades, valve components...One will remain rust free, while the other one may be dusted with rust, near welds, near machining processes, near areas of polishing or buffing. Stainless is easily superficially contaminated when in contact with iron. Buffing a welded or machined surface with a wheel that's previously buffed mild steel will often cause the stainless to rust. Sandblast media that's previously cleaned mild steel will also cause stainless to show rust. Again, it's superficial, so it's easily removed, but just like cancer, if it's not completely removed, it will show signs of rust again.
In industry, it usually didn't matter. If it did become a concern, additional attempts to remove the rust via oils or solvents and polishing with wheels that have never been in contact with mild steel eventually stopped the rust.
In the case of a firearm, if the iron contamination has embedded itself deeply, and if serious attempts to clean the rust off the stainless is not successful, then I do believe the manufacturer has a responsibility to make it right. Finding the "right" customer service representative at Ruger or elsewhere, may require multiple attempts. There are certainly some customer service representatives that consider it their mission to save the company money at any expense, including convincing the customer the defect is good enough, or is somehow the fault of the customer.