Let's talk about Diadem mechanics

[alimdia]

edit: also Star ranks spawn rate seems to be around 15-20 mins

It's entirely random, otherwise you would see at least 3 bosses per duty, I've had runs with only 1 boss.

I've had it where an Emergency Mission popped as soon as the group entered Diadem and also another time where it popped 5 minutes after entering.

Keep in mind just because your group just entered Diadem doesn't mean there weren't other groups already farming.