Reproduce: open user search from Social menu. Type in a name and wait for results. Click Social tab. Click on the magnifying glass icon.
Window is now cleared of results, but search term remains above. If you click the search term and try to enter again, it will not return results. It will not return results even if you backspace and retype, as long as the string is the same as the previously searched term.
Typing a different term, or going to the social tab and clicking user search from there will start a new search however.