This is unfortunately a somewhat well-known bug that has been in the game for close to a year. Not exactly sure which patch broke it, but I've seen complaints about this dating all the way back to a few months after the game's release.
If you have access to an older save file you can get the regular ending by ensuring Fane never meets or interacts with the small voidwoken bug that appears just before you enter the room where the final fight takes place. Don't ask me why or how, but Fane meeting the aforementioned voidwoken appears to be what triggers the bugged ending.
