Is this in direct sunlight, or everywhere? My S24 Ultra will become dark after a few minutes in direct sunlight, this is due to the S24 overheating, as the first several minutes are fine. This is not caused just by the phone per say, but also by the phone absorbing all the light that is hitting it. If you keep your phone covered or in the shade, then it will stay bright for longer periods of time. This is assuming that your issue IS in direct sunlight.
