I have created a custom list which contains around 32,000 items. There is a 4 part key which looks something like 'T1-0000-X-XX'. Each part has a meaning. In order for users to be able to filter by each of the parts, I separated this key into 4 separate columns and indexed each column. Call them Key1, Key2, Key3, Key4. When a user tries to filter on Key1, it does not show all the values that exist in the list. They start typing and the one they are looking for does not show up. Therefore, SharePoint does not allow them to use that value to filter. I know the value exists and if I choose another value and then change the URL so it filters on the value I actually want, the filter works. So it is there. But no matter what I have tried, I cannot get the value I want from the UI. I have not yet tried recreating this field yet, which might be my next option. But repopulating it might be a hassle. As you can see in the image, T2 is clearly available in the Group field. But cannot be found by the filter.