The Great Vault Fault

So i done 8 Tier 11 delves and when i opened the great vault the last slot is showing a tier 8 item, why is this? and what can you do to fix it?

Doing Delves higher than 8 does not improve your Vault.
Only benefit of doing 11s is getting Guilded Crests from first 3 Tier11 delves.

so i cant get 649 in all 3 slots? it says in the 3rd slot 623 it will be based on the lowest of the top 8 runs this week, but i done all 11’s, is it because i done the gobfather at the start of the week and he’s a world activity and world activities count as i tier 1?

I might have missunderstand you since you said you had “Tier 8 item in last lost” and it sounds like Heroic item.

As for wrong item in last slot.
One of my firends had similar issue … did exactly 8 delves but then missing last slot.
I had similar one … I could swear i did 8 delves, but I did check before reset and count shown 7 … so I did 8th and unlocked my last slot.

I don’t daubt you ability to count the delves you did … but did you check your vault progress before the reset ?

I wasnt missing the 3rd slot i got, 649 in the 1st and 649 in the 2nd then 623 in the 3rd, and last week i made sure i only done tier 11 to see if i can get to chose 3 slots of 649 gear instead of 2 as i was doing the first 3 for the crests and the rest tier 8 just to do the great vault

Then you likely didn’t complete 5 t8 runs and 3 t11 runs, maybe 4 t8 runs and 3 t11 runs and then a world event (remember its delves and world content) counting as the 8th activity, but at tier 1 cause its a world event, forcing a t1 reward in the vault.