With how buggy the rest of the expansion has been I can't help but think Blizzard has actually forgot to enable uldir cache missions to actually function! They *exist* so why aren't they obtainable? The only one you can get is the normal/heroic from emissary. BUT they have LFR and Mythic versions for Uldir and not just that but they are also being made for the next raid.
Uldir caches: https://www.wowhead.com/items/name:uldir+treasure
Next raid caches: https://ptr.wowhead.com/items/name:zuldazar+treasures