Tried GetDetailedItemLevelInfo() on two of my characters with legion items. One is 110, the other is 104.
API works fine with 110 character, but it seems bugged for the 104 one. The latter one got a ilvl 730 boot from NORMAL Eye of Azshara at level 103, but now the first return value(claimed effective ilvl as tooltip) is 740.
And some other bugged values:
A quest reward cloak, tooltip as 729, API returns 739.
A 1/2 upgrade heirloom, tooltip as 463, API returns 605.