I think it's a matter of habit for many, not because they don't know the difference.
I think you mean nationality/ethnicity.
You are correct. There was a massive war between japan and korea and to this day they still hate each other because of the stuff that japan tried to do korea (abolish their language fully, there are still grandparents and grandmas still suffering to do this day because they can't return to their hometown) so calling korean manhwas 'sensei' will get on their nerves, especially when the authors have family of their own who went through suffering.
More than me not wanting to say it right, I haven't quite memorized which is which, so I might have made a few mistakes a few times. I think this might be true for more people.
I do want to say it the correct way though, so I'm sure I'll memorize it in time!
One question to help that a bit though; how do you address Korean and Chinese authors then?
Idk why it’s bothering me so much but some of y’all need to stop calling manhwas “manga” becuz it’s not. Manga is what you call Japanese comics. Manhwa is what you call korean comics. And manhua is what you call chinese comics. Plz stop mixing them up becuz it’s not proper to call something korean a Japanese term and vice versa. Just like how you shouldn’t call manhwa authors “sensei”. That’s not right. It’s not the same race nor culture. As someone who’s very knowledgeable of the Japanese and korean culture, they are both very different in so many ways. I know it’s such a stupid pet peeve that no one will bother taking into consideration, but I’ve seen a lot of Koreans online taken it offensively when people call manhwas “manga” or call their authors “sensei.” It’s just not culturally and racially right, you guys.