This is a great question. There is not an intuitive way to undo the Assumed Answered, but there is a workaround. You can mark a reply as the Correct Answer, which will change the state of the Question from Assumed Answered to Answered. Then, you can unmark the reply as Correct Answer, which then reverts the thread back to an Unanswered Question.
I've added a reply to your thread which you can use to mark as Correct and then unmark. Once complete, I'll remove my reply.
Qlik Community Team