Today at about 11:40 CST, I was in an instance of The Chant to Cleanse the Calamity (Magatsu AIS battle) and we got to the third phase, but the golden Magatsu never spawned. It showed the usual "Press O to view event" thing, which pointed my camera up at the sky, but nothing happened after that. Judging from the chat in the instance, it didn't spawn for anyone else either and we were forced to wait out the timer.
Then when the timer hit 0, again nothing happened. About a minute later it finally dismounted us and dropped rewards.