Harald I believe what you are referring to occurs when you have opened or viewed an app, but not interacted with it. What I mean by interact is select text, open a ink, press an interactive button. If you paste text into a document, then slide towards the left to vanadium, you would be able to slide back to the right back to the document. Now slide left back to vanadium, select some text (the interaction), and you will now be only able to slide left as you are already viewing the application you interacted with last.
I hope I am describing this accurately, it might seem confusing but it definitely seems by design to me.