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.
I was a bit offended by tourists doing that mmhmm thing until I found out that it’s considered polite in the US.
I was interpreting it as “yes I know you are thankful to me, and so you should be! By the way, I’m an oaf.”