Videos from youtube, tiktok and other sites don't show the full screen when i play them.

Hi, for info, I’m not asking how to watch a video in full screen.

My issue is that often, when I play a video from YouTube or TikTok etc, it’s like half or a third of the screen is cut out. For instance, I could be watching a duet but I can only see one person because the second one is just out of view even though they’re both supposed to be on screen. (I can just about see the arm of the other person). Or if I’m watching football, someone off screen will pass and then I see the ball come into the screen. But obviously I’m supposed to be seeing the person passing the ball too.

i have no idea why this is happening but it’s been happening for years. I currently have an iPhone 15 pro max so I don’t know if it carries on from phone to phone as I keep the same iCloud stuff. It’s happened continuously over the last 4 or 5 years and I have no idea how to fix it. Any help would be much appreciated. Thanks.