Do you have some custom CSS for the app? E.g. a remote theme in Site administration > Mobile app > Mobile appearance > CSS. Just to discard that the button is hidden due to a side effect of some custom styles.
No, that field is to be able to add some custom styles for the app (for Premium sites only), but in the past we've found people that removed some elements by mistake using CSS, that's why I asked.
Right now I don't know why this could happen, it might be a bug in the app. I'll send you a private message with some instructions so I can test it myself and try to debug the problem
Once it's integrated you can apply the fix in your site until a new minor version is released.
There is a workaround to make the app display the button: the quiz should include a question that isn't random and it's supported in the app. But please notice that the user will be able to submit the quiz in the app even if it has unsupported questions, we usually display a warning but because of this bug we cannot know if the random question is supported by the app or not.