12 mini screen dimming

My 3 month old iPhone mini is useless in the sun. The screen dims so dark I cannot see what I am taking pictures of. If I am in the car and researching on safari after about 10 minutes the screen dims so dark it renders the phone useless. After 4 troubleshooting calls the problem IS NOT auto brightness, not white point, not zoom filter, not manual brightness turned down. Apple techs logged onto my phone and changed all the settings that could be an issue. Apple exchanged the phone but new phone has exact problem. Two more calls to apple, they said update to latest software which had just come out. No change. Trip to Genius Bar for diagnosis, no help. Another call to Apple and another reboot, another diagnostic still no resolution. Apple needs to address this. Wish I had my 6s back.