Killing Modniir Ulgoth in the place that the achievement describes does not grant an update to the collection in the title. Second time killing this boss without any updates. Would hate to make this an urgent fix, but I have been working towards this since 2 years back.
If you need any other details let me know!
~ Tehrak Hiwhay
EDIT1: I tried for a third time doing the pre-events AND turning auto-loot off and it worked.