Step sixteen on The Missing Diplomat questline flags you as soon as you accept the quest.
I should note that this quest is not marked (PvP) like quests such as “The Final Message to the Wildhammer”. I should also note that you get flagged as soon as you accept the quest. You do not need to attack Private Hendel. Also, Private Hendel is PvP flagged while friendly, before you accept the quest, but loses his PvP flag once you accept the quest.
I believe this is a bug regarding quests where the objective is to attack a PvP flagged NPC. Two other quests that had this same mechanic (that were both fixed) were the lvl 10 Human Warrior quest “Beat Bartleby” and the lvl 31 Alliance quest “The Attack!”
I have captured a recording where you can see that I am not flagged before accepting the quest, Private Hendel is friendly and PvP flagged before accepting the quest, as soon as I accept the quest I become PvP flagged, Private Hendel loses his PvP flag, and that the quest is not marked as (PvP). Missing Diplomat PvP Flag - YouTube
Edit: I should note that this issue is critical because of the resurgence of popularity for the Hardcore game mode.
There are multiple people who abuse this bug by camping near the area on their multilife characters in hopes of ending someone’s one life character.
Edit 2: I should clarify I am specifically referring to Classic Era. I realized after reading other topics that this forum is mostly about WotLK Classic.