More brightness is more heat, more heat is more risk
A very bright screen also emits heat, and we have a summer in which we know well what it means when our iPhone gets too hot. When the phone’s sensors detect that the temperature has reached a certain level, one of the measures it takes is to lower the brightness of the screen to stop giving off so much heat.
That is why on days of heat wave, using the iPhone in the sunlight means that you have a hard time seeing the screen. And more if you have it in a casesince then it is more complicated for the heat accumulated inside to dissipate.
Is there any way to avoid this? You can mitigate this effect turning off auto brightness on the iPhone, which you can do in the Accessibility section of Settings:
If you deactivate that automatic brightness you can regulate it as you see fit, but note that the system itself already gives you the warning: abuse that can affect screen performance and even battery life in the long run. Therefore, I recommend that you use it in situations where you need to see something important on the screen.