I microwave with the bag in. You want nucleation sites during heating so bubbles can form. If you ge over 100c with water in a smooth container, then add tea, that’s how you instantly boil over.

That said, I’m in the camp that wants their black tea just under the boiling point. It’s less bitter and doesn’t need cream or sugar. I know that a certain time, cup size, and power level usually sticks the landing between 95c and 100c.