It's essential to use formal and honorific speech (높임말) or polite speech (존댓말) when meeting someone for the first time. The same applies when talking to strangers, teachers, co-workers, bosses, and in any situation where you want to be polite. But then why do native Korean speakers sometimes use casual speech (반말) when meeting non-native Korean speakers?
It happens online, as well as in person. Has this ever happened to you? What did you do? What did you say?
I talked with my friend about what we've done, and what we recommend doing - as well as what we recommend not doing if this happens to you.