Having same problem still. Wasted alot of time doing approx 15 of the level 8 delves but the last one still shows Tier 1 on the 3rd slot… the GMS say we need to post here BUT I dont see ANY answer as to how their going to compensate us for all this time wasted trying to get that 3rd slot to read right.
I had this same issue (did 10+ T8 delves, last slot was still T1). I asked some party members about it and one suggested doing a T2 delve to see if it upgrades from there. I did, and it did. After it upgraded to the T2, I went and did a T8 and it upgraded to a T8. So, it’s a stupid round-a-bout way to fix the bug, but it works.