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.
It happened again. I pressed "mark as wrong" during the review of failed words and nothing seemed to have happened so I pressed it again, still nothing. When I continued with the review the marked word did show up again. After the quiz it says I have 1 left to study but if I press the button it says I have nothing to study. Just as the last time. This time I only have a 90 day streak though so not as sad :P
I have a few master schedules that I study. If I complete 100% of the questions every day, a very satisfying counter gets increased at the bottom. When I used the "mark as wrong" button on the 18:th, my mastery schedule dashboard(?) showed that I had 1 left to study but if I pressed the study button I was told there were no reviews to do so I couldn't keep the streak going. But no worries, I remember that it's over a 1000 :P I just mentioned it in case that helps finding the problem easier. That it's not just visually showing 1 left to review. The graph that shows the number of questions per level also had 1 red left, but not even on the correct level since it moved up during the review.
It's called "Kanken" (id: 97136) and the 2 times I had 1 review left undone because of this bug were the 18:th this week and then around the 22:th of april this year. I think it said 911 days then.
It seems to me that it works fine if I only press the button once so the bug might just be that the button does not disappear after being clicked which enables scattered minds like mine to schedule the same word for review twice or whatever.