If you search for games in the lobby while filtering by Single Duel Mode, it will search Matches instead. I tried searching while filtering by Matches, it returned nothing.
Bug reproduction steps
Pick ‘Single’ in the ‘Duel Mode’ dropdown and search for games in the lobby. All found rooms will be Match games.
Screenshot OR error code
Searching for Single games should return Single games only. Searching while filtering for Match games should return Match games only.