Quested yesterday in Azshara on my main (lvl 50), logged in today, finished 2-3 quests and went to the mailbox to receive the mail about 16th anniversary.
Proceeded to acitvate the buff, start the quest and went back to questing.
Soon after, all NPCs in Azshara disappeared, friendly, enemies, even other players. When I left for Orgrimmar, the NPCs at the northern gates respawned as I entered the city, and despawned again once I went back to Azshara.
Abandoning quests (in Azshara and from events), toggeling wm on and off, joining other groups,…nothing fixed it for me.
As far a I know other zones are not affected, and I only met one other player with the same issue.