I was raised and educated well inside the Anglosphere (USA) and was also taught to write formally in that way.
Do the people who say you sound like AI give you any specifics?
Also, if you don't mind, what was your English education like? I understand that quite a few Americans work in South Korea as teachers but I have no details about how that manifests.
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.