I am a colleague of Sheno, and we were finally able to test in our Dev environment to see if this was linked to the Adaptable theme or not.
This is our test page with the Clean theme:
Same page in Adaptable:
When we had Adaptable in Moodle 3.2 in our Dev site we had no issues, but since upgrading to 3.3 the navigation to move to the next section is missing, as are the options for other sections in the Jump to... menu. As far as I know no changes were made to the Adaptable theme settings that would change this, but if anyone has any ideas?