Hi temiloluwa1,
Thanks for reaching out in Apple Support Communities. I understand that landscape orientation on your iPhone is not changing fluidly as you expect while watching videos. I can help point you in the right direction.
You mentioned being on iOS 13, there's a recent version of the iOS software that is now available. I recommend making sure that your device is updated to the latest version. This helps provide software improvements and enhanced security updates on your device. If needed, here's a helpful article on how to update your iPhone: Update your iPhone, iPad, or iPod touch
If the issue persists after updating to the latest iOS, follow the steps in this article: If the screen doesn't rotate on your iPhone, iPad, or iPod touch.
I also recommend making sure that your apps are fully updated. To check, you can open the App Store, tap on your photo on the upper right corner and follow the prompts to update your apps.
Kind regards.