Its the same bullshit that's been going on for ages.
Young actress grows up, people think they have an obligation to look and act how their idea of the now adult actress is rather than them being themselves.
It happens to pretty much all young performers, but girls get it worse because sexism and patriarchy amplify it.
She's damn near 21 now. She's married and moving into an adult life and some people are idiots. They expect her to either stay that little girl, or at least not act adult in public.
I was so confused by this post, and it turns out I was conflating some celebrity I'd never heard of with a Julie Brown (who was famous maybe 30 years ago).
Then I googled Millie Bobby Brown and am confused again: She looks like a typical early 20something, which she is?