Font substitution does not work for glyph fallback

A mix out of Arabic and Japanese glyphs should be rendered with two different fonts which are chosen. For example the Arabic glyphs should be rendered in "Arial" and the Japanese glyphs in "SimHei". So set font substitution of Arial to "SimHei" with QFont::insertSubstitutions("Arial", QStringList("SimHei"));

Expected result is the Arabic glyphs should be rendered in "Arial" and the Japanese glyphs in "SimHei", but turns out that Japanese glyphs are rendered in "MS Gothic".