Bold and other font weights will show up as regular if the font family is not found in the system.

Text {
        id: t
        font.family: "NotAFont"
        anchors.centerIn: parent
        text: "foobar"
        font.bold: true
    }

The text here will not be bold, but removing the "font.family" line (or replacing it with a font family that exists) will fix it.

This seems to be related to c9eff4.