Arbiter of Ash drop?

Hi! So i finally killed uber arbiter and had no boss-related unique drop. He dropped only side unique (Mask of the Stiched Demon) and arbiters reliquary key. is it expected so the key's drop replaces boss-uniques? Or is it a bug?

If i remember correctly, in PoE1 reliquary keys are kind of a random drop which droppes as an addition to a boss unique. Is it different in PoE2?