I always think the tea ferments in the flask and tastes much stronger than in a pot. It stews, if you like. My top tip is never to add the milk in the flask. Always carry it in a beaker or something and then add it. Conversely, instant coffee tastes quite good.
It's oxidation, not fermentation. The flavanoids in the tea oxidise up, the tannins settle out and the whole thing tastes like liquid leather. Take the hot water in the flask, then carry the tea bag with you.