Advanced filter is not returning results when filtering on a specific value, clearing that criteria and searching for 'All' does return results.
The Advanced filter returns no results when specifically filtering on a value where the 'All' criteria does.
The value being filtered on is likely to be a customer created value which may contain a 'space' or hidden character.
In the Workspace configuration menu, select the work item type and field in question. Edit the field in question and completely remove the value and re-enter it correctly. If there was a hidden character involved this will clear it out and the filter should then find all items that contain that value.