Hey Nethareal,
First things first, overall it sounds a lot like you are looking at a previous week’s reward where you didn’t claim them yet. The Great Vault will store unclaimed rewards for a long time, and you’ll need to actually claim these repeatedly in order to get to the most recent reward from the week before.
Secondary to that, while a Mythic Artificer kill will count for the “kill X unique bosses” progress, a single Mythic kill does not count to get you a Mythic level reward from the Great Vault. The rewards from the three raid boxes will be based on the lowest difficulty you managed to get an eligible kill for - if you killed 3 heroic bosses and 1 mythic boss, you’ll still only get a heroic-level Great Vault reward.
In any case it sounds like you have a ticket going, so I’m sure this will be looked into further and you should see a reply as soon as possible.