In my experience this issue is due to the screen cable becoming disconnected when the screen is opened.
To prove this, open the screen only a little and turn on the laptop.
Now open the laptop further and the screen will turn off.
The screen can be turned back on by using the relevant key board shortcut.
If the screen is still open then it will immediately turn off.
Lower the screen to form an acute angle with the keyboard and try the keyboard shortcut again, the screen will remain on.
The issue therefore is that the power supply to the screen from the motherboard is becoming detached as the screen moves from an acute angle to an obtuse angle relative to the keyboard.
This can be remedied by opening the computer using a torx screwdriver and placing an object such as as a ball of duct tape on the motherboard over the screen’s serial data cable to prevent it from lifting off the motherboard as the screen is opened.
The reason that the login screen is visible is that it draws less power than the desktop or browser window, as the cable remains partially attached it can supply sufficient power for the login screen yet the bright white of a browser window can not be powered.
Disappointing on ASUS’s behalf, the cable should secure to the motherboard via another method, the current design is not fit for purpose.