QFont::setFeature lacks documentation on font fallback behavior

What happens when the requested font is not present and Qt falls back to some other? Will the feature get applied to that other font, too? For instance, I want to set ss03 feature on a font for alternate presenting of some letters. If the end user enters some character not present in the font (e.g. a CJK character), font fallback will take place and the meaning of the feature might be completely another in that system font (altering other letters). I still want font fallback, so NoFontMerging isn't an option.

I imagine, there are also use-cases to apply features even after font fallback, though?