Just to chime in on what @chirmer said: Searching by tag and filtering by tag are by design different in Things 3. If you search on a tag, you will get every item not completed that has that tag. That means items on the Upcoming (Scheduled) list and on the Someday list as well as on the Anytime list. If you want currently actionable items with a given tag, you should filter the Anytime list.
Ohh, good to know! I didn't realize they performed differently. That makes sense, thanks!