See
this kind of thing is exactly why I really dislike when they try to replace Korean honorifics with English words, it doesn't make any sense why he'd accidentally call her miss or be embarrassed about it.
I feel like it would have been better if they'd left it as nuna and slapped a little tl note in there for those who don't know what that means. I know it doesn't match the European-esque setting, but it doesn't make
sense if you translate it to miss or sister. It doesn't carry the same meaning.