Sometimes on Youtube, when my phone returns in portrait mode, the app is cutted ...
I would recommend you to clear cache and storage of YouTube app. This solution helps to solve a lot of issues with apps.
No, this is a know issue which we are yet to find the root cause for. It can happen in other landscape apps too. Go to home screen and back again and it will look normal
It returns normal as soon as I touch the zone of the nav bar for information. And it seems to happens only with gesture navigation.