> As for Kamala Harris and her sudden southern accent in front of certain audiences...

I grew up in one place with a very marked accent, and moved later in life to a place with another distinct, very different accent. My accent shifts within minutes of talking with people from either place. Sometimes I'm in a room with people from both places, and my accent changes depending on who's in my field of view. This doesn't take any effort; in fact, I need to make an effort to avoid it.

This is how accents work. Our brain adapts very quickly to try to fit in.

(I grew up in Buenos Aires and now I live in Madrid)

I have made a similar observation, maybe even stronger: In Lower Austria, I speak the dialect, but only 50km away in Vienna, people speak in more of a "standard German" way, I'm guessing due to the large influx of German migrants and because the dialect is considered lower-class (by the younger generations especially, who, probably due to social media, speak very Germany-esque) and I've noticed I myself also switch to a "standard German" type dialect.