When the buff dialogue for Head of Onyxia, Head of Nefarian, Heart of Hakkar, or Head of Rend is started on one layer, it should start the dialogue on all layers where the relevant NPCs are alive. This functionality was added in July 2020.
Rallying Cry of the Dragonslayer just went out on Classic TBC Whitemane and did not start the event dialogue on other layers even though the NPCs were alive.
Please re-apply the hotfix from July 2020 so world buffs go out across layers.