iPhone 12 Pro Max brightness issue.

I bought an iPhone 12 pro max back in February. I noticed the screen was not as bright as the others by comparing it with other people’s iPhones. I contacted apple support then sent me another one then the new other one has the same issue. I went to an apple store then they did diagnostic. They told me the secret is not good so they sent me the third one and it’s the same issue. The problem is they keep sending me phones from the same place which is Tennessee. I really need help. I been with every apple technician to try to solve the problem but no luck.