Iphone screen dims on Photos App and on Facebook/Instagram reels

my iphone dims by itself when I browse pictures and videos on photos app. Same also as in preview on camera app. It also dims when I'm in facebook and instagram reels. It dims when I'm watching reels from both apps. But, watching video posts, or stories, it's normal. It doesn't dim. It only dims on this iphone (iphone 12). But in my sisters phone (iphone 13), the dimming is not there, it's normal. What have caused it? Is it software? Becauce we both my sister have the same IOS version.