I think probably Microsoft messed up with this. Probably? Ha!

I still have the original Xbox360 but the servers have closed already for a couple of years here in Europe. So the result was that I got locked out of all my local accounts too. No way to access those save games, can't login. Because it appears that an Xbox360 account can be either "pure local" or "online", and of course mine were "online" because I used to buy stuff in the store. Now the closed the servers and "online" accounts won't log you into Xbox360 locally without logging you into Xbox Live first, which is impossible.
I did find information on the Microsoft website concerning this. It amounts to "Suck it up, Princess - time to by an Xbox One".
However, I can still play Xbox360 games on the console, but I had to make a new account, local only, carefully avoiding any attempts to connect to Xbox Live, because that will lock it.
I understand from Renee that you have a better experience over in the US, these things are managed regionally. But it looks like may be it is coming to you too.
Anyway, since you are on Xbox Next Gen, you should not have such issues. But yeah, I am not surprised they messed up with the DLC! They probably figured it's an old game, and everyone is buying it on Steam or GOG now...

<rant over/>