Prev and Next button appear after revisit the quiz.

Another person on my team (Dane Boyington) has commented elsewhere on issues we have using custom navigation. It's just very frustrating how much relies on automated features that can't be accessed or replicated through custom navigation. To have the default navigation appear when the developer has expressly disabled that feature could compromise an entire course. The default navigation won't have any custom triggers applied to them that the developer may require. In our work, we use an "IsQuestionActive" variable to determine whether the user has interacted with a quiz in a particular way. We disable default navigation to ensure that this variable has integrity. If Storyline's default navigation can appear in certain situations, then the user could bypass the custom navigation we've setup with its attached triggers. This would make our "IsQuestionActive" untrustworthy, and could cause the user to get trapped on a page (which is ironically the situation you claim this behavior seeks to avoid).