Almost All Bittersweet Endings are Downer Endings.
After some reflection, almost all bittersweet endings are really downer endings. They all have some sort of irreversible loss in them. Would you even recognize what a bittersweet ending should look like? A bittersweet ending would be a Croatian man dying, waking up, and finding himself isekai'd into a world where, him, his family, and his race occupies an analogous position to the Yamaguchi family and the Empire of Japan in the present world. And that isn't even a golden ending. You can barely conceive of that. That's how bleak and shitty your universe is. Hint: Back to the Future Pt. 3 was the closest humanity got to writing a golden ending (only loss was Biff's fate) and you hated it !