Heya, I also didn't receive my AC when I bought some earlier today. If you have more than one Microsoft account then your situation may be similar to mine, I have two Microsoft accounts one for gaming and one for business and it seems I was logged into the business account in the Microsoft store when I made my purchase even though I play the game from my Xbox live account that has my gamertag.
When I started the game I of course selected my gaming account (at the title screen) since it holds my gamertag. But when I clicked to buy AC I was brought to the Microsoft store where I was already logged in as my business account. Without a thought that maybe I needed to change my accounts in the Microsoft store client I went ahead and bought the AC. Now it seems this account that will not be playing the game has received my AC. I'm not confident that support will be helping me move the AC to my account that actually is playing the game.