Plunder Harbor, an eastern area of Tiragarde Sound, does not appear to be considered as part of the zone. After many dozens of solo PvP kills (killing blows included), no Assassin buff has been granted. It appears that travelling in to or out of this area reset the counter. This affects quests such as Servant of N’zoth.
Plunder Harbor is the prime area for Alliance to find Horde in Tiragarde. This prevents using it for PvP.
Edits for clarification: my testing included zero deaths, no phasing, no hearthing, no queues, no parties, no raids, nothing. Half a day wasted trying this quest. I’ll have to try again elsewhere.
When leaving Plunder Harbour, it pops up “Tiragarde Sound” as the zone you’re entering.