My mother used underwhelmed all the time, so I never thought much about it until I started seeing it in common usage in a sarcastic way. but that’s even a lot of years after her death, so...
People in other countries say things that, when one thinks about them, are either totally logical, or makes more sense the what we term the “correct” phrase.
It reminds me of a story where an American was going to a meeting in the UK. In the office building he asked the door man where the suite was he was looking for. The door man replied, “It’s just down the hall, past the lift, on your right.”
The American replied, “You mean ‘past the elevator,’ don’t you?”
The door man said, “No, sir. I mean the lift.”
The American said, “We invented the thing. I think we should know what it’s called.”
The door man, still as unflappably polite as only Brits can be said, “You may have invented the thing, sir, but we invented the language.”