I see. Thank you.
When I wrote documentation, I found this style. It seems to be different between American and English style.
So in English it is correct in some style. But hard to understand. (And I see it was by accident.)
The German sentence need a closing mark nevertheless.
I just found a context were in German the question mark is correct.
4. "Bist du sicher? Ich hätte die Tickets vor dem Konzert kaufen sollen?" (It indicates that you are in doubt about an answer/proposition.) (Rückfrage, it is similar to the original question.)