I had this exact same issue with the same shirts, the shirt had like 4 print locations so each time it got worse.
We got credited back the cost of the shirts because I think it is a factory dye issue, and even running at a low heat you still risk getting the spots. I didn't do any testing, but possibly just going through a consumer washer and dryer could bring out the spots.
I made the customer change shirts, he wanted to just do a different color, but we compromised on a different tee.
