iPhone 12 mini screen dims very dark when plugged into charger

Hello, I was having a Issue with my iPhone brightness going very dim. I followed some recommendations to turn off auto brightness, turn off true tone but was still having the problem. I have diagnostic done at Apple Store, then I have screen replaced at Apple store then I have everything else replaced at Apple store and I finally realized I am only having this problem when the phone is plugged into the charger ( original 5w or 18w). Which is often since I am constantly running apps and using the gps. It dims to a level that is not ideal. While I can still see, it’s just barley visible and uncomfortable. Is this a known issue?

Problem was reproduced on the brand new iPhone 12 mini at the store.

Apple representatives are not experienced enough to help me resolving the issue.