First check if Luxi Sans font supports Chinese glyphs.
Qt tries to use different font if some characters are not supported by given font (I heard it is called font fall back system).
It is possible that they change/improved how missing characters are resolved (by which font) and this is why you see a different fonts in those Qt versions.
IMHO it is better to use font which supports required characters/symbols.
If you like one font in English localization and another font in Chinese localization then set font in code using translatable string as a font name, so you will provide different font name for each translation.