I got the iPhone 8 plus (iOS11.2.6) three weeks ago, and found a damaged pixel, and still on the street at the temperature of the surrounding +5 degrees, the flash does not work.
What to do about it?
iPhone 8 Plus, iOS 11.2.6
Get help with the camera on your iPhone, iPad, or iPod touch
Your flash isn't working
Use Control Center to test the LED flash. Turn the flashlight off and on. If the LED flash doesn't work as a flashlight, contact Apple Support.
If the flash isn’t working, or it goes off every time you take a photo, check your flash settings. Tap
in the upper left of the screen to choose a different setting.
If you take extended video with the flash on, or you’re using the device in a very warm environment, you might see a message that says the flash is disabled. The message remains until your device cools down.
When the flashlight is turned on, the flash works, and also works when the photo and video are used in a warm room.
I suggest you do an experiment if you have a smartphone of this model: put it for 5 minutes in the refrigerator and try to make a photo with the flash turned on.
Even Apple Authorized Service Provider and Apple Retail Store recognize this problem but can do nothing.
A LCD display I'll try to give a diagnosis, let's see what they say.
iPhone 8 plus problems with the screen and flash
