I've beaten the game 11 times, and i've check and beaten it on all action and all riddle difficulties at least once each, so why is extra riddle not popping up?

I heard you set both action and riddle on hard and it auto unlocks, but this doesn't work either and the save comes up as "hard" instead of "extra"

am i doing something wrong??

Are you sure you still have end-game saves present on your console or memory card from beating it on all 3 riddle difficulties?
If so, I don't know why it's not coming up for you. I haven't actually tried for it myself. I mostly prefer "normal" difficulties, occasionally hard.