Please can I confirm some basics before investigating - apologies if these do seem very basic, but if we can rule out the obvious to start with it does make problem solving easier
1. Please can you confirm what course format you are using (Topics, Weekly, or a 3rd party installed format, etc)
2. Check in the Clean theme to rule out theme issues (I don't think this is a theme issue, but this will eliminate that)
3. Ensure Editing is turned off - in most/all course formats, when editing is turned on, then all topics are shown
4. Check if the page you are on is the course index page - On my Moodle3.1, Topics format, Clean theme, when I select one section per page then I am presented with a page listing the topics: clicking the topic title takes me to the 'one topic section per page' display.
Richard