The theory is that Costa Magica which is going to join the Carnival fleet later this year would do Miracle's cruises while Miracle goes to Australia.
On the same day that they announced Miracle's dry dock, a message went up on the Carnival Australia website that a new ship will be named for Brisbane soon. It would make sense for Miracle to take what was originally Spirit's place (before it went to US) because they are in the same class so it would be easy to transfer to bookings over.
Miracle had its last dry dock in 2020 right before the pandemic, so it could have been due in 2023 but I guess it needs one earlier.
It is also possible that Miracle could go to Australia, then another Carnival ship takes its place, and then Magica takes that ships place.