It's now well into the afternoon and still no new challenge is available. How come?

It's a bit of a mess, thought, isn't it? I see that some seasonal characters have finished all their seasonal objectives, which include doing three challenges. So clearly, a part of the players have their new challenges available on Monday morning, while others are still waiting.

Isn't this supposed to reset globally for everyone at the same time? New challenge available on Monday 9 AM or whatever? How is this even coded, if the game somehow divides the players into groups with various unlock times? And why?