"Submit" Isn't Advancing Quiz Slide

After answering a question in a quiz, I normally click the "Submit" button to advance to the next question.

But today I made a minor change to a quiz and was testing it to see if it worked.
I answered the question and clicked "Submit." Unlike in the past, a blank screen was displayed. I discovered if I click the "Play" button in the Playback Control toolbar, I can get to the next question.

Is this a bug or did I accidentally change a control that is making this happen?