I think this world quest is either bugged or was just bugged for me. I had joined a group to kill it but upon killing it I didn’t get the world quest completed nor the world quest reward. Now the quest was still showing as active for me on my map but now the NPC was without it’s silver rare spawn emblem, & instead now had the elite gold emblem adorning it’s avatar. I spent several minutes waiting trying to figure out if it was just a delay in the game & would be credited once the delay caught up. When it didn’t I joined another group to kill tUmbra’rix & this time got credit for the World Quest as well as the quest reward.
Now the World Quest is no longer showing up on my map that it needs to be done, but I found it highly strange that I didn’t get credit for the first kill. But I did want to bring it up because “technically” I just did the same World Quest twice. The first time I got an Azerite shard from it. The second time I killed it I got faction reputation as well as the quest reward, but no Azerite shard.
FWIW, it’s actually a currently common bug to not get WQ credit for a rare kill WQ, even if getting kill credit and loot.
There seems to be a situation where killing “group sized” rares (such as Umbra’rix) will not grant WQ credit in a raid. Quest credit seems to only be scoring in normal groups or less, including shared tag by individuals.
I opened a thread on this topic spanning several elite rare WQ mobs a while back. There are other threads complaining about the same bug.
I wish they would fix it, because at the moment I am skipping most of them if I cannot solo it. It happens often enough it is not worth using the group finder for the chance it might grant me credit.