As far as I know Samsung isn't a Korean family name, it's just a brand.
That said, are you sure it wasn't the + that caused the problem? I've run into that a few times, presumably when someone tried to roll their own email validation.
It's probably a specific policy of Samsung which doesn't allow the word samsung in recipient addresses. I had the same issue, but with samsung@private-domain.tld
Sometimes you can sign up with the + but when you try to log in either on the homepage or an app, the login is invalid because of that + sign. Different validations. Stopped using that way after getting locked out of accounts 6 months later...
My usual "smasung" typo worked fine when I registered with them. I use a service for disposable addresses redirected to my main mailbox for potentially spammy registrations which I don't really care about, instead of just creating new accounts which is way too inconvenient to manage.
I'll make these intentional letter swaps every time just to avoid regexes and automatic filters.
I don't know what they are thinking. Isn't it a real family name in Korea?