On the results page, above the table, you can see that the quick search only looks for open issues ("Status: UNCONFIRMED, NEW, ASSIGNED, REOPENED"). 16640 has already been fixed, so you have to include the RESOLVED status when searching.
You can do that by clicking the "Search" link to the left of the quick search text input field. There, select "Status: All". Then you get these results which include 16640:
https://issues.dlang.org/buglist.cgi?bug_status=__all__&content=__FILE_FULL_PATH__&list_id=219081&order=Importance&query_format=specific
Closing as invalid.