iphone 13 pro max screen is more than a half grey and rest is white

Did you do a normal restart or a force restart? A force restart normally fixes the issue. If not it could be a hard ware issue. Its best if you contact apple support and show it to them-under warranty they will fix/replace the device

https://support.apple.com/contact

Force restart an iPhone with Face ID

To force restart iPhone X, iPhone XS, iPhone XR, iPhone 11, iPhone 12, or iPhone 13, do the following:

Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side button. When the Apple logo appears, release the button.

It won’t delete anything, don’t worry.

Force restart an iPhone with Face ID

To force restart iPhone X, iPhone XS, iPhone XR, iPhone 11, iPhone 12, or iPhone 13, do the following:

Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side button. When the Apple logo appears, release the button.

Try a hard rest or force restart like I explained above. if nothing works contact apple support