How to enable full screen video output on iPhone 15 Pro Max via HDMI?

I’m using an iPhone 15 Pro Max with iOS 18.3.2.

When connecting via USB-C to HDMI (using Apple’s official AV Multiport Adapter), the video from apps like YouTube or Camera is displayed in a small window with black bars, instead of full screen.

This didn’t happen with my iPhone 12 (Lightning model), where videos were automatically displayed in full screen via HDMI—even when the phone was held vertically.

It seems that the HDMI output method has changed on USB-C iPhones. I would like to see an option or fix that allows full screen video output on external displays, just like older iPhones.

Please upvote if you’re experiencing the same issue!

[Re-Titled By Moderator]