Locking screen in portrait mode

[Richt811]

Hello

I have had my Sony Xperia XA1 for a couple of months now. All that time it has been successfully locked in portrait mode, which is what I want. Somehow it has become unlocked. Now my apps switch between portrait and landscape which is most annoying. How do I switch it back please? 

[Qatrium]

@Richt81

If you are running Android Oreo, it should be in Settings -> Display -> Interactions -> When device is rotated -> Stay in portrait view