Why do my colors occasionally change (e.g. dark gray becoming lighter)?

This started to happen on August 3rd on my old iPhone 11. It happened 2 more times within 2 weeks, and early September potentially. I was already set on getting the iPhone 13 Pro before this even happened, and that is what I did as soon as pre orders opened. However, even on the OLED display, I encountered the problem twice on my new phone. My iPhone 11 was on iOS 15 beta when those happened, but it is happening on my 13 Pro on the latest stable build. When I encounter this problem (if I notice that the colors look a bit different), I would open a game that has dark colors in some areas to confirm that this occurred. If I can tell that it looks off, I would restart my phone, which resolves the issue. I am hoping that this is an iOS 15 bug or an issue that others have as well, because I got my iPhone 11 in late 2019 and never experienced this issue until August 2021. As this is a display thing, screenshots do not work as they change back to normal too when I restart.