Stanley Thermos Makes Coffee Sour After Three Hours Replacement Request

I received a Stanley thermos bottle with cup as a prize at the Standerton Skou. From the beginning I noticed that when I make coffee for my husband to take to work, the coffee becomes sour after about three hours in the flask, whether I use fresh milk or powdered milk. Because of this, the product is not usable for us as a normal thermos for work, since the coffee cannot stay good for a few hours. We are very disappointed, especially because this is a Stanley product and we expected reliable quality. I kindly request that this Stanley thermos be properly inspected to determine the cause of the problem and that it be replaced with a properly functioning product, as we cannot use this one for its intended purpose.