That used to happen to me more often. When I first came to HN, and even now if I am not careful, my comments can get flagged. Also, when I translate from Korean using DeepL and paste the result, people often say it sounds flagged, awkward, or unnatural. I studied English more seriously in graduate school, although I dropped out. In Korea, there are quite a few Americans who teach English. Public schools often have native English-speaking instructors, but in my case I learned English more seriously at graduate school, and universities also make students study English almost semi-compulsorily.

In Seoul, there are probably many teachers who mainly teach middle and high school students, but a lot of that is through private education rather than the public school system.