I didn't see it myself but I've read spoilers for The Book of Henry and A Cure for Wellness, with both films have some pretty dark detours in their direction after some point. Although for A Cure for Wellness, it's less dark and more insane as to where that finally ends up.
As for movies I've seen, I'd consider Sorry to Bother You, Hancock, and The Sound of Music as examples. I'd also say Gremlins goes dark even before that Santa Claus story TBH, with how the gremlins start attacking people. That you never really see what happens to some of them really puts it in the "dark turn" territory, even if some of the events veer more towards "black comedy" rather than explicitly "dark".
Coco also has an great example with the reveal of a certain character. Even if one figures there was something shady going on with that person; I honestly was not expecting it to have it shown to play out as it did, let alone veer into that territory to begin with.