Okay, so I have a possible explanation for why some people (myself included) may not be seeing their codes. This is a pain in the a*s to explain, so bear with me. I actually discovered that I did get my Founders code (though, as of yet, not the Black Dog pack).
I ordered the game on the MS Store on Windows. I have an account there, which we'll call "account 1."
I also have an xbox account, which we'll call "account 2." These two accounts are not the same, but they act like it. In other words, it's entirely possible to play on your "account 2" while actually technically only having the licenses of "account 1" active.
If you have multiple MS accounts, make sure that the account that has your codes activated is the account you are signed on to on the MS store -- if the account on your MS store and Xbox app aren't the same, then you will not see your loot.
It's stupid this even is the case, because as of right now I am signed into the MS store on an account that doesn't even have the game, but on the Xbox App I can still launch the game because it is installed (and I do own it, of course).
So, people, be very careful when you are activating your codes to have the right account logged on in the MS store and on the Xbox App.