The "Stranger Things" star recently addressed this issue head-on in a sit-down with TikToker Max Balegde -- where she was plugging her new Netflix movie, "Damsel," and where she was asked directly about her accent constantly switching up ... albeit, in very fawning terms.

Watch ... Max prods the subject, but does so in a way that's already taking Millie's side. He says he totally gets the notion of someone's accent changing depending on where they are.

That's the rationale a lot of her fans/supporters have offered up in defense of Millie after an interview she did with Jimmy Fallon nearly two weeks ago went viral ... where she sounded like a straight-up Yankee, despite having risen to fame with her signature British accent.

MBB took the reins in the convo here and explained why her accent changes on a dime -- BTW, during this conversation with Max ... she sounded like her usual red coast self, almost exaggeratedly so -- and basically said, yeah ... my accent changes with my surroundings.

She notes that she bounced around as a kid in terms of where she grew up -- including living across the pond, as well as here in the States -- and now, she's living in NYC with her fiancé, Jake Bongiovi ... so she says she's adopted more of an American accent lately.

That's all fine and good, but what Millie also admits is she'll tweak her accent depending on who she's talking to -- as a courtesy, it seems, and to fit in better. Which is ... interesting.