If you only have available a custom field as a course filter, then the filter is empty (nothing is preselected).

How to reproduce it (4.3 site is like that but you need to adjust some things):

• On browser as an admin
  • Courses > Default settings > Course custom fields: create a custom field (for example a dropdown menu)
    
    
  • Plugins > Blocks > Course overview: on 'Available filters' deselect everything except 'Custom field' (block_myoverview | displaygroupingcustomfield) and on 'Field to use' (block_myoverview | customfiltergrouping) choose the custom field you created on the previous
    
• On the app as a student, check that the selector on My courses doesn't preselect anything