Group by view not showing all groups

Hello, 

I have a document library with multiple tags. One of the fields is a dropdown with 3 items. When I do a 'group by' using the dropdown field I can only see groups for 2 items. The 3rd one is just invisible. I have documents tagged to all three options. So all the 3 tags are in use. When I ungroup them they are all visible.

Any help will be appreciated.