Apple iPhone 12 Pro Max temperature warning?

In the last week my iPhone has had the temperature warning come up while it was on charge in the house, while it was charging in the car with a usbc, while on the internet and while using instagram and all these happen while I was in ac room or car. I had software ver 14, saw 15 and 16. I updated to 15 and straight to 16. I feel like since these updates I never had this issue before. On the backside on the phone where the Apple logo is what gets hot. My battery percentage is at 87% . Checked battery diagnostic with Apple and they said no problem. And I feel like the phone battery dies very quickly. I’ve removed any apps not being used as well. What can it be?