8 plus 256gb screen goes dim by itself what can stop this
Please find out why the 8 plus 256 is having screen dimming problems
iPhone 8 Plus, iOS 11.2.5
I would check that nothing (case, screen protector) can obstruct the ambient light sensor.
Shouldn't be this case, go to Settings>General>Accessibility>Display Accomodations and check the auto-brighness.
If it's on (green), turn it off, restart your iphone, then test it for a while (the screen brightness shouldn't dim).
If you want to use autobrightness, then turn it on, test it (the brightness should dim when in low light), and restart your iP8 again.
Read also this article:
Adjust the brightness on your iPhone, iPad, or iPod touch - Apple Support
Regards
Giulio