iPhone 13 Pro sleeping/locking issue

My iPhone 13 Pro goes to sleep/locks itself EVEN THOUGH I’m watching YouTube videos in full screen through Safari. This shouldn’t happen as my screen was in full screen and video was still going!

My iPad Pro doesn’t do this at all.

 This is not related to auto-lock or brightness as my iPad and iPhone have the same setting for auto-lock and brightness, but my iPad doesn’t lock itself whilst I’m watching YouTube in full screen through Safari.

There is a way to fix it for iPhones?