An update to iOS has caused some devices to lock their screen rotation without notifying the user. If this has occurred you will not be able to view video in full screen mode until the lock is removed. To view this setting swipe down from the top of your screen on iPhone 10, and up from the bottom on older phones. The image below shows the screen rotation lock on.
Tap the icon to turn the lock off. You should now be able to view video full screen.
Please contact Apple support for additional information:
