QLabel::setFont() strange behavior

QLabel::setFont() has different behavior depending on how and when  it called. It works correctly only after selecting font using QFontDialog, in other cases (font was deserialized from config, for example) - it looses some font attributes even these attributes are present in deserialized font.

Applying deserialized font (read from config) from constructor looses `styleName` font attribute, but doing the same on button click (but NOT doing it from constructor, i.e. label has default font) lead to visually correct effect, but QLabel::font() reports NO styleName field set...

see comments in attached sample app and screenshots.