I was watching a video from two years ago about different social norms and this showed up. Found someone questioning the same eight years ago on reddit (when it seemed less normalized). It feels so weird not being aware of this shift, even as a foreigner.
We have our “c’est moi” in Brazil as well: “eu que agradeço”. Honestly, mmhmm is what seems alien to me.
It sounds absurdly rude if not sarcastic to me, I think I’d rather even a sarcastic “you’re welcome” than that because at least it sounds like the person acknowledged your gratitude. Really, if you’re gonna use “mmhmm” you might as well just stay silent.