YEs it gets heated up faster with the app open and use under the sun. Once it heats up the software will do auto adjustment of screen brightness by lowering it down until it cools down. And there is nothing you can do to adjust the screen brightness as long as the phone is still hot. This means whatever i do on the phone while im outside has to stop because it is hard to see whats on the screen under the sun! Such an expensive phone expects its user to wait until the phone cools down then can go on with the work! What a terrible experience!
Try to switch off the adaptive battery toggle under app power management. I suspect this causes the battery drain in note 20 ultra. After switching it off seems there is a little improvement. I dont really trust the software capability in learning the users phone usage pattern.