Steps to reproduce:

• Switch the active theme to Classic
• Create a course
• Add multiple sections to the course and label each section individually
• Go to any section page by clicking the section title on the main course overview page (you should end up on a page with the URL /course/section.php?id=<courseid>
• Have a look at the dropdown widget which is located at the bottom of the page

Expected behaviour:

• The dropdown widget contains the list of all sections (except the one which you are currently viewing)
• You can browse to all other sections by selecting a section from the dropdown.

Actual behaviour:

• The dropdown widget contains just one section (it seems to be always the last section of the course except when you are on the last section page, then it's the second to last section)
• You can browse to just this single section by selecting it from the dropdown.
   
  Notes:
• As this dropdown navigation is only shown on themes which set the
  

  $PAGE->theme->usescourseindex

  property to false, this only affects Classic and not Boost.

• This bug was first reported on https://github.com/moodle-an-hochschulen/moodle-theme_boost_union/issues/639 as a Boost Union bug (which was not the case).