As others have also reported, The Great Vault isn’t tracking tiers from delves correctly.
I began delves on this character (other than the story) this week.
I have unlocked all 3 loot choices, choice 1 being tier 5 (completed TWO tier 5s).
So, I have completed, at minimum,
1x T1
1x T2
3x T3
1x T4,
2x T5
By that logic, I have done at least 7 delves at T2 or higher, yet my second option (Complete 4) is still Tier 1, where it should be T3 (from completing T3, T4, T5, T5).
I did a T8 yesterday. Vault is still reflecting a T1.
I have done multiple T8 delves this week and last and not gotten Vault credit for them.