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I have a similar problem with this set I bought from Amazon, but the spots on mine do come off with a scouring powder (like Bar Keepers friend) or scouring pad.
Like yours, it’s only the knives and I believe the knives (or maybe just the knife handles) are made from a different (and probably cheaper) stainless steel because they are thicker.
The staining on my knives is galvanic corrosion (aka dissimilar metal corrosion) that occurs when the knives are laying in the stainless steel sink for hours while they are wet and I see evidence of it in the sink. It may also be occurring in the dishwasher while simply touching other pieces of silverware.
We also got spots on our butter knives like this. They came off with a paste of Borax (but not anything else we tried). We let the paste sit for 10 minutes or so and then wipe with a microfiber cloth. Worked for us. It’s worth try.
Take it back to the store. Many stores will take back merchandise if it seems defective. I just took back some bath towels to Kohls. After washing them just once in cold water, they looked like rags. Kohls took them back and gave me a store credit.