(Looks like a number of folks are suddenly having trouble with req packs…)
Purchased the Halo controller and entered the included req pack code but it never showed available in game. Other req packs have worked fine. Contacted MS chat support and they can confirm it was entered and valid in my account and should be available. They sent me here saying this is the place to get a response and the issue corrected (after going through the whole cache flush, reset account and restart routine).