Long before Shen Yuan opened the Secret Realm Square-Inch, he surreptitiously arranged for Gong Buyu to stay behind in the Imperial Capital as a backup plan.
As the Secret Realm Square-Inch was personally created by Shen Yuan, it was one thing to deceive other people, but there was absolutely no need for Gong Buyu to enter it as well.
Moreover, during several encounters, Shen Yuan had vaguely discerned Emperor Xia's true nature.
This Great Xia Emperor's desire to regain control was so strong that it was almost undisguised; with all Grotto Heavens Blessed Land cultivators heading to the Secret Realm Square-Inch, Emperor Xia would certainly take the opportunity to make his move and prepare for the grand City God Festival.
Once the Capital City God Festival preparations were complete, Xia Yunqiu would lose her last bit of value, and Emperor Xia would definitely make a move against her.