Now after finishing video, if u go back to video details page/ overview, it's in landscape mode even though my phone is set to portrait mode.
It should be that the overview page to be in portrait even though I watched a vid in landscape.

Tap the Back button. It will rotate the phone back to portrait mode.
Using the cross button should also make the app in default portrait mode. It's a minor bug n the tab is present in the bottom of u click the cross button.
Could you change your issue to say that instead, then? Thanks.