I also have an iPhone 14 Por Max that decides to black out also. Sometimes it's in the middle of the day and sometimes when I take it off the charger in the morning as it did today. The phone is still on, and Siri is active. I've activated her with the power button and asked her to turn the brightness all the way up. She said she did it, but it doesn't get brighter.
With all that said, I've found the easiest way to correct the problem.
Activate Siri with the power button and tell her to turn the phone OFF. She will verify your request then turn it off. Wait about 5 seconds then hold the power button to turn your phone back on. All will be back to normal.