Quiz Next Button and Prev Button not working correctly

we have discovered an issue where, the Next and Previous buttons has disappeared.

our students have seen, if they have done their last question and want to check their previous question. the button will disappear in question 20.

and also, after going to the last question and backing, the Next question button also disappear.

you can access the quiz section by following the steps below:
1. Login(Test account) to https://cityacademy.sg/
(Username : ryangoh1621@hotmail.com, password: ryangoh1621@hotmail.com)

2. Proceed to https:https:https://www.cityacademy.sg/quizes/accounting-trial-exam-mcq-b/
(just copy the link will do after login in)

The page I need help with: [log in to see the link]