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The announcement is a set-up for Sana to be able to return whenever. She's doing a "soft-graduation" until she recovers this time next year. Her back issues have been improving but she won't be prepared in time for when Council gets their 3D, so she's doing this as a way to avoid dragging them down. Sana will be the first alumna to return and has the full support of management for this, which is why the Collab Gauntlet feels so different compared to Coco's.
The girls were sad at first not because she's graduating, but because she'll miss Council's big moments due to unfortunate events that impacted her health.