Whenever a new Heavenly City Fragment entered his Inner World, it would be licked by the Inner World and lose "something". As for what that something was, even Sister White Dragon and Senior White could not figure it out.
However, every time that "something" was licked off, the scholarly faction's world of the golden lotus was no longer picky with the fragment and would be willing to take it in.
So now, when the Heavenly Emperor mentioned that something was missing from the fragments, Song Shuhang immediately thought of that.
He could also infer from the Heavenly Emperor's tone that that "something" was probably very important to her, possibly even more important than the Ancient Heavenly City Fragments themselves.
But knowing all this, he definitely could not admit to what had happened.
He could not admit to it even if he was beaten to death!