The release notes for the iOS 17.1 beta says that it contains a fix for an issue that might be the cause for the scattered reports of iPhone 15 screen burn-in.
Burn-in seen on an iPhone 15 Pro Max (source: "Surfphysics" on Apple Support discussion)
The release notes are clear and succinct. In the update notes, Apple says that the iOS 17.1 update "Fixes an issue that may cause display image persistence."
A month after the iPhone 15 range went on sale, users of the iPhone 15 Pro Max were reporting issues with screen burn in. Despite the fix, it's not a widespread problem, and it's not known whether it also affects the iPhone 15 Pro.
Burn-in may be best known from the old days of CRT screens, but it has been an issue with OLED displays. In any case, burn-in is typically something that typically happens after a very long time with the same images being shown on the screen.
It's not clear what Apple has done to fix the issue. Presumably, it's related to the always-on display, as images shared have typically been with the phone off.