iPhone 15 Pro screen dims after periods of rest

Do you have that Always On Display turned on?

• Always-On display is on by default for iPhone 14 Pro, iPhone 14 Pro Max, iPhone 15 Pro, and iPhone 15 Pro Max. Your iPhone display can operate with a refresh rate as low as 1 Hz with a new low-power mode. These models use improved technology that allows the display to dim when you lock your device or leave it idle.

It can also be turned off as seen in this Support Article

Adjust the brightness and color temperature on your iPhone or iPad - Apple Support

Also, an additional feature is to dim the screen when it gets too warm, which may be the case when you are keeping it in your pocket, and then when you take it out you will see the brightness increase as it cools down. That one cannot be turned off and the purpose is to keep your phone from overheating.

• If your device gets too warm, the display might dim and your maximum screen brightness is reduced until your device can cool down.

mrchntmarine wrote:

so the issue happens whether kept in pocket or not..... I checked the above setting - it was toggled off. I did toggle on then unchecked the options it controls, then toggled back off. We'll see.... Do you know of any bugs as it relates to this? Im not sure when to check for that one. Tks for answering !

I have seen reports of the screen dimming, but also seen the same reports with iOS 18 so I do not know if it device specific or even iOS specific. The only reliable documentation from Apple is that it will dim when the device becomes warm and I gave the pocket scenario as just one possibility that may happen. There are other times that the device will become warm where that could happen, such as background activities from apps, iCloud backups, and iOS downloads, although usually the last 2 only happen when the device is plugged in. If you would like to speak to Apple directly, the best course of action may be to contact them by either phone or chat using this link.

Contact Apple Support - Apple Support