Heat while charging.
My iPhone 13 Pro Max heat up while charging.
iPhone 13 Pro Max, iOS 16
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Some activities such as charging or a lot of computational activity, generate heat. Some warmth is to be anticipated.* Read this article about keeping your iPhone, iPad, or iPod touch within acceptable operating temperatures - If your iPhone or iPad gets too hot or too cold - Apple Support
If you think there is a problem with a device, have Apple inspect it. Go to this page to find your nearest Apple Authorized Service Provider (AASP) or Apple Authorized Distributor (AAD) --> Find Locations
How to make a Genius Bar appointment
1. Go to Genius Bar Reservation and Apple Support Options - Apple
2. Go to "Get hardware help".
3. Select your hardware and continue selecting options requiring replacing parts until you see "Bring in for Repair"
4. Sign in with your Apple ID
*"Your iPhone might get slightly warmer while it charges." - from: How to use your MagSafe Battery Pack - Apple Support