Reproduction

1. Create a book with a version
2. Create a page named "page1"
3. Save it
4. Edit the page (which edits the versioned content sub-page) and change it's title to "Page 1"

Result

In the navigation panel, the page is displayed with "page1".

Here is an example with also sub-pages.

Expectation

The navigation panel should display "Page 1".

Not having the title used for the current version/inherited version creates different issues:

• it displays pages in a different order than they will be in the published space. For example, on master space they will be displayed "Page 2, Page 3, page1" and on the published space they will be displayed "Page 1, Page 2, Page 3",
•  it can make the user feel lost. He cannot find the currently displayed page in the panel, so doesn't know where he is currently in the book. 
• the user might not know where to go because he can't see the title he is looking for.

Resolution

This could be fixed by changing automatically the title of the book page and other versioned content sub-pages when one versioned content sub-page has its title changed. I don't think this is a good option. Having different titles for publication depending on the version is a nice feature to have.

I would advocate for modifying the navigation panel to display the name of the versioned content sub-page associated with the currently selected version/inherited version (if one is available) instead of the title of the book page itself. A default case would have to be considered if the page has no version or no inheritance, I guess that simply displaying the book page title works in this case.