Thank you for your asking me questions.
I'm not sure of your troubles as we haven't stored matcha mixed with milk powder, but the problems of lighter green, bitterness and loss of scent are all phenomena that occur when matcha was stored too long.
Air and light may cause this trouble, but the main cause for the deterioration of matcha is temperature.
I advise you to store in the fridge in order to make the shelf life 1 year.