Mine also has this problem - I just hanged up with Bose customer support.
There was a known manufacturing defect, and an amount of these headphones are affected.
I gave them the serial number, they confirmed, that although the headphone is outside of the warranty period, since it belongs to the defective ones, they replace it with a new one at no cost.
I will get a shipping label, and in 10 days, a new headphones with 1 year warranty.
I needed not to present original packaging or proof of purchase.