Players in WoW Classic have found a bug where sapping Overlord Runthak, Major Mattingly, or Field Marshall Afrasiabi during their even dialogue will prevent the Rallying Cry of the Dragonslayer Buff from being applied. This bug was not present in original Classic WoW, and is going to be hotfixed soon, according to a blue post on the WoW Classic Forums.
Apparently, devs had made some changes to how the buff is applied in the code to better optimize server performance. Once the hotfix is applied, they will still be able to be sapped, however it will not stop the buff from going out. After the quest that triggers the dialogue is turned in, players of the opposing faction will still have the opportunity to stop the buff from going out by killing the NPC before the dialogue is complete, which is consistent with how it worked in the original game.
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While we typically try to avoid making changes that can affect legitimate PvP interactions including players engaging in combat with faction-specific NPCs, this particular issue is noteworthy because of some under-the-hood changes we made some time ago to how the Rallying Cry of the Dragonslayer buff is applied in both Stormwind and Orgrimmar to improve server performance.
Without getting too deep into the nuts and bolts of what we changed, the important thing to be aware of is that being able to sap Overlord Runthak, Major Mattingly, or Field Marshal Afrasiabi during their event dialogue to prevent this buff from going out was not actually possible in original World of Warcraft. Sometime very soon we will be applying a hotfix that will more accurately reflect the correct behavior from original WoW.
These NPCs will still be able to sapped and otherwise crowd-controlled as before, but following this hotfix these effects should no longer prevent the event from playing out and the buff from being applied. Please note that the opposing faction can still attack and kill these NPCs after the quest is turned in but before the dialogue completes and this will interrupt the buff going out. This behavior is correct and has been preserved.
Thank you for the reports about this!