You go to the expense of buying loose leaf tea and then leave the leaves in to steep for >5min which basically makes teabags taste better because you can remove them and you can get quality tea in teabags nowadays (not just fannings and dust). I know different types of tea need different length of steeping but no black tea can survive longer than 5 mins. The reason you can still drink tea that was steeped longer when you make it in a cup is because the temperature in an open mug goes down very quickly. In your setup with a closed mug it's going to be even worse.

You must've seen this coming.