I was likely some COBOL system with UI displayed by some miracle on a modern PC with windows. The employee probably didn't even know what encoding is, they didn't intent to question your identity.
Of course, but, to give you a sense why I felt so annoyed, the conversation went something like this:
> What is your last name sir?
"de Bakker"
> That name is invalid. What is your name?
My last name is "de Bakker".
> That name is invalid. You need to give a valid name.
"de Bakker" is my last name. It's on my passport.
> Sir, if you don't cooperate I'm gonna hang up and proceed with the next customer. What's your name?
So it was very clear that I was the one at fault here.