Marking the last question in the quiz wrong doesn't reload the quiz to redo the wrong question, it takes you to the finish page. A parallel behavior is with the mark as correct feature, where marking the last question right if it was wrong reloads the quiz to take you to the finish page. (reload means the grey overlay and the spinning thing, not the browser reload)
I really like this feature. It's very seldom needed but really great in those few times I need it. I tried it today. It seems like the level counter got stuck. What I did was that I failed that word normally during the quiz. Then when I went through my failed words after the quiz I marked it correctly and then pressed the mark as wrong button.
I'm pretty sure the word was 縁談 but it's level 5 now, not 6. :-)
This is what happens when I press the study button:
I managed to make it happen again. What I did this time was I clicked "mark as wrong" but the quiz only reloaded and the button was still there so I clicked it again. The quiz just reloads and the button was still available so I continued with the quiz. The card was correctly marked as incorrect and showed up again but now i have 2 red words after I've completed all (exactly like the picture I sent here earlier but 2 instead of 1).
I don't know if this is relevant but when the word showed up for review again I failed it normally. So during the normal review I clicked "mark as wrong" twice, during the first review of missed words I failed it and then the third time I got it right.