Checking the bug report “similar topic”, it seems this is an ongoing bug.
I decided to start and clear the achievement Algari Delver Healer (1,2,&3) last week. I had never healed a delve before starting this and ran them all on my resto druid. After starting at lower tiers in fungal folly, I quickly realized I had the gear/ilvl to be able to heal Brann through tier 11s. So that is what I did. Last week I completed 11 of the 15 delves for the achievement at tier 11.
Yesterday I opened the vault rewards on my druid to see that the World(Delve)'s third reward was only a Veteran 1/8 ilvl 668 piece. The reward description for that slot clearly states “The item level of your reward will be based on the LOWEST of your Top 8 run this week.” Since I ran 11 tier-11 delves, the LOWEST of my top 8 was tier 11, meaning I should have received a 694 Hero 1/6 piece in all 3 reward slots.
And of course, the only item actually usable for me was in that third, lower ilvl slot.
I put in a ticket, I attached relevant screenshots, and this is the reply I received: “The GM team cannot generate reward options or exchange Great Vault loot.”
We all know that’s crud. I am asking for the item that I received as a reward to be given to me at the PROPER item level. And I am asking devs to fix this ongoing bug. I bet it has something to do with completing world quests screwing up the tally.
edit: relevant ticket with screenshots of issue can be seen on character Acirex-Suramar US.