Hello! My friends and I just watched The Secret of Moonacre, and we were wondering if these exposed bustles in the dress were ever a thing. I wasn't sure what to do other than a quick google search, but I've been following you for a while, and hoped you might have some insights!
Not that I'm aware of! In the era this is riffing on bustle-wise (mid-late 1880s, looks like), great pains were taken to hide the lines of the bustle beneath the skirt. In fact, earlier satirical cartoons mocked the way "ignorant" maids supposedly wore cage crinolines by showing Visible Hoop Line:
(The Way To Wear 'Em, March 1864. Cage crinolines/hoop skirts were cheap and universally popular, which made some upper-class people very nervous about a potential narrowing of the symbolic gap between Haves and Have Nots.This manifested in classist satire, often tinged with anti-Irish prejudice in the US since many Irish immigrants worked in domestic service here.)
The hooping-wire used to create such understructures was meant to be hidden by a petticoat between the hoops or bustle and the outer skirt.
You do sometimes see much earlier Spanish farthingales worn as overskirts with decorative, colorful "ribs":
(Salome from the St. John Retable, Pedro Garcia de Benabarre, c. 1470-80)
but I don't think that's what the movie has adapted for its costumes
of course, it's a fantasy not set in our world at all, so the inaccuracy doesn't bother me. but there's the history behind it, since you asked!