I have beaten Halo:MCC on legendary and have the achievement to prove that I have but when I get on Halo 5 I still have not gotten the armor.
Try this:
Check the customization menu in H5. Not your req collection. It should be there. If it is not. Load MCC, go into one of the menus and customize an emblem or something. Reload H5 and check your customization menu.
So this happen to me as well like 3 months ago or so. I beat whole game and I checked my game mission status on here to make sure every level was done on legendary. it was. so what’s the issue? then I checked My achievement and it said I earned 49/50 for the “legend” achievement. 49/50 basically finishing all the missions. so I replayed the last mission on H4 from the beginning and once I finished, I received like 10 achievements or more including the “legend”. I went to H5 and it was unlocked. double check your achievements and mission difficulty. i used to use the Helioskrill Warbound. now I keep the original equipped. I didn’t go through all that not to show it off.