I played quite a while this morning and received a few new treasure cards (pretty good ones too!) When I logged in just now I found them missing- has this happened to anyone else?
They're not missing, I just verified they're all on your account - however Treasure Cards do not automatically get equipped, you must go to the Treasure Card tab in your deck and place them into your deck. (I also noticed you have more than one deck - make sure you're placing them in to the one you want to use).
Next time you log in, when you're not in a duel press P - then click the 'gold' Treasure Card button at the top. All your Treasure Cards will show up on the right and you must click them to add them to your currently selected deck.