Hello there,
We have made a test here and it’s properly working. Your theme is adding extra previous or next button onto quiz. It’s working properly when swithing to one of default WordPress themes, Twenty Nineteen.
As for the “logic” part, hidden questions for each visible question has to be allocated on the same page. It won’t if the hidden questions are on another page. It can be one visible question that triggers multiple hidden questions, but they all have to be on the same page. The next set of visible question and related hidden questions will have to be allocated on the next page. Doing it this way, you will have one visible question on each page.
This is a screencast that shows the quiz working properly.
https://drive.google.com/open?id=1ly7han-X0ddPAllsmHpYJ6JXXB2nFKGc
In conclusion, our plugins are working properly. It’s an issue with your active theme and how the quiz is setup.
Regards,
Kharis