Why is losing an accent to sound American ‘natural,’ but gaining one abroad ‘pretentious’?

Americans: A common trope is the American person lives internationally and comes back with an accent. The joke is that they are trying too hard.

Buuuttt… We don’t think that when someone is from another country and drops their accent for an American one.

Why is that?

Even on a smaller scale: If someone from the North returns with a Southern twang, it’s weird. But if a Southerner loses their twang, no one bats an eye

If you think about it, it would make sense that someone actively living in another country, with another culture and accent, would pick up accents and customs. It would be their everyday life, so it makes sense that they have developed the accents or cultural practices.

So if people who migrate to the US develop American accents and fold in American customs tot heir own. Why wouldn’t the same be true for Americans that live (long term) abroad?

Why is one seen as natural and the other as pretentious?