When used with Book Module, Book Outline doesn't work properly

We are going through a pretty big site and we are doing it with V5-beta of Quiz module.
Our quiz pages are part of a book, and we found a bug on the Book Outline section.

When creating a new quiz, and selecting the book into the outline section, the ajax stall and keep spinning forever. Image attached.

We bypassed the issue saving the quiz without book outline, then we went into the outlined tab, assigned only the first level to the quiz node, saved again, then went to the outline page and re-ordered with drag-drop the quiz in the right section of book.

Let me know if you need any other info to replicate.

Cheers,