It has been reported that it is not possible to filter for people who have a particular tag in a database. In this case, the tag was added to existing profiles by an import. Even though the tag displays on profiles in the database, when a filter is run for people who have this tag, the filter displays no results.
This behavior is not consistent for other tags within this database, so the issue appears to be limited to a specific tag.
We are aware of this issue and actively working on a fix.