Trivia - whelmed means in-water. So it’s technically reasonable to speak of being underwhelmed, whelmed, and overwhelmed. For whatever reason, in the U.S. we only use overwhelmed unless we’re being sarcastic.
Also, in India they talk about preponing a meeting. It means moving it earlier. Don’t ask me why we don’t do that.
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.