FontConfig matches can have an 'embolden' property to control auto-embolden. Even if it's set to false, Qt ignores the request and emboldens the font. (And viceversa, if it's set to true, Qt doesn't embolden.) Basically, the auto-embolden logic in the Freetype font engine is controlled by the font weight and the request weight and some other environment variables, without taking Fontconfig's properties into account.