iphone Xs Max running iOS 12.0.1 ( also happens on iOS 12. Screen still brightens randomly then dims after I lock it. I have rebooted it ,Turned it on and off ... same issue occurs .
Same here, on my CS Max – these steps worked.
https://appletoolbox.com/2018/09/why-is-my-iphone-xs-xs-max-xr-display-so-dark-a nd-dim/
Same exact problem with me. Brand new iPhone XS Max, screen gets really dim for no reason. True tone is off, night mode is off and auto-brightness is off. Still extremely dim. If you take a screenshot the screen flashes to the highest brightness setting and the difference is very noticable. I erased my phone and reset it and that fixed the problem for about a day and now back to a dim screen at full brightness again. Talked with apple support and they were absolutely no help at all. Not sure if this is a hardware problem or a software problem but it’s extremely annoying for this to be happening on such an expensive phone.
Go to accessibility settings in there there’s something about auto dimming, Once you find the setting in there your screen won’t dim anymore
iPhone Xs Max screen dim and brightens on its on randomly . ( auto brightness and True Tone are turned off)
