I just killed the bosses for the 3 weekly raid quests from Nessie from Hailol and I only got quest completion for the Venific Locus boss. Tons of people from the 4 raid groups shouted that they didn't get credit either. And it was completely random who got the quest completion, not even based on which raid group you were in. I'm from the shard those bosses were killed in, (Zaviel) so it isn't a problem with cross-shard raids either.
I did get the sourcestone/planarite rewards from killing the bosses, but no quest completion.