On Japanese Windows, if a configured font misses glyph, it fallback to default font (Maybe "MS Gothic" for Japanese). It's common behavior on most editors. I found that random Hangul can be shown too.
Most programmers use far better aligned font like Source Han Code JP, or merged and aligned font like Myrica. So I don't know much. I don't recommend mixing fonts for coding that prefer monospaced font, but it should have a fallback.