When you are in an app like safari in landscape mode and then use multitasking to select an app like messaging, shouldn't that app have already corrected itself to be landscape too?
I'm including this pic to try and explain what I mean.
I've seen that bug for years - going back to iOS 7. I've always just found it amusing that the app thumbnail sometime didn't rotate, and like Paddle1 says, you just tap it and move on.