Is there a name for the trope where a story is high fantasy at first glance, except for it's not fantasy and is actually set in a post-apocalypse dystopian future?
I'm embarrassed to say that I have encountered this, this particular type of story on multiple occasions... So I got curious, is there a name to this trope?
Seriously. There's a dozen links to TvTropes and almost none of them match OP's description, but they're all upvoted to high heaven. Not to mention the unrelated replies talking about their favorite stories which don't actually match the trope either.
The TvTropes links are mostly right though? It matches the third variation of Earth All Along. The linked examples match what OP is describing except not being restricted to Fantasy.
Well OP didn't specify Earth. I can think of some stories where it transitions from high fantasy into sci-fi but is not set on Earth, which is definitely not under the 'Earth All Along' trope.
Spoiler:
The Gods are Bastards is the first one that comes to mind, where it turns out the planet is a colony in an unstable part of space that's been quarantined by the original science team who have ascended.
Pity, 'cause it's a great question, and a great trope. I can think of a few good examples. Maybe it's time to start a TVTropes account and get editing.