My Brightness lowers significantly when I open any games or apps

I dont know why, but after leaving it off for nearly 24 hrs it suddenly fixed itself. No problems as of now

Okay I would try on another TV just in case and then if it does not work a full reset (without keeping data)

I reset the Xbox but it came with option to keep my data on the Xbox, I clicked this option.

Hi,Nick! I'm Luis, a fellow Xbox Gamer . Thanks for reaching us out here.

Try resetting your display settings

If there's a disc in the console, eject it.

On the console, press and hold the Power button on the front of the console for 10 seconds to turn off the console.

Press and hold the Power button and the Eject button until you hear a beep to turn on the console. You’ll hear one beep right away and a second beep 10 seconds later. The power light will flash on before the second beep. Don’t let go until you hear the second beep or see the console start up in low resolution mode (the Xbox startup screen will appear slightly pixelated).

Note If you have an Xbox Series S or Xbox One S All-Digital Edition, press and hold the Xbox button  and the Pair button  instead, as your console doesn’t have an Eject button.

In either case, this will start up your console in low resolution mode (640 × 480).

To change this setting, press the Xbox button  on your controller to open the guide. Go to Profile & system > Settings > General > TV & display options, and then select the resolution that you want from the options under Display. For more info, see:

How to change the TV display settings on your Xbox

If you’re required to complete these steps on every startup, change your TV connection to HDMI as shown in step 3 of the section below, "Your screen is blank, and you use an AVR in your setup."

So you factorr resetted the console?