I've heard that rewards vary depending on which characters you pick in the multi-character chapters & which ending you get... but I have not verified this, so it could be completely false for all I know.

Just mentioning it because, if true, that could explain how it is you beat the game, but didn't have a particular reward. *shrug*

Chapter 12 is only Liu Kang and i get the Skin between other rewards, but dont have in unlocked in collection. Icon and Background for Ch12 i also had in Kombat Kard, but they both dissapeared, now they are locked between other locked Icons and Backgrounds. and if i do multi-character Chapters later, i dont get any rewards at all, its just for completion. i replayed Ch12 again, although with skiping the cutscenes and didnt get the Icon/Background back. will try whole Ch12 later, that only what i think about doing now.