For example in the 'Microsoft YaHei' font, the Unicode cmap has: <map code="0x7f" name="space"/>.
By way of contrast, there's no such mapping in 'Noto Serif CJK SC'. U+7F gets you the .notdef(?) glyph in that font (box with an X inside). Switch the font to YaHei, and all the boxed Xs are replaced by spaces ...
There must be a good reason for this mapping. Anyone have an idea what it could be?