I don't know if it is better or worse now, but don't expect the same legal processes as in the US or UK. Don't expect to get a phone call. A friend is married to a Korean woman. Out of the blue during a visit, she was arrested, detained and hair was taken from her for drug testing. The family couldn't get to her or get information about why she was taken. It sounds terribly frightening. Fortunately, she'd never done drugs and they were able to get her out after a few days.

So, certainly do not try buying or smoking there.

Since they ask for fingerprinting, I think the charges would turn up in the report from the US police department that the Korean government requires for the visa. I don't know if misdemeanors show or not.