Not quite the same thing, and probably unrelated to you and the other folks who got the Moon Lord spawning immediately, but I know after a lot of running around on my test world (used mods to indicate EoC and EoW down, before I realized the dude doesn't show up on my lifeform analyzer) I randomly saw "King Slime has awoken!" It was so sudden since there'd been no slime rain or even a lot of slimes that...I got startled, forgot to watch where I was going, forgot I took fall damage, and died
He never did spawn on me, but maybe that's because I died. I don't know though--could the update and whatever the "this thing has started!" status messages go with the new event mess with boss triggers?