This might have happened because Senior Yellow Mountain and the others had not expected Song Shuhang to be unsatisfied with four cores, and still get a fifth core. Or perhaps they had not expected that Song Shuhang's strength would undergo a qualitative change after he got the fifth core.
It appeared that according to the speculation of the seniors of the Nine Provinces Number One Group, as long as Song Shuhang reached the Sixth Stage, the danger of him self-destructing due to his cultivation technique would disappear.
They had set it such that the safety seal which they placed would break after he reached the Sixth Stage.
However, none of them thought that while Song Shuhang was still at the Fifth Stage—after he condensed a fifth core—he would attain a qualitative change to his spiritual energy and divine sense that rivaled that of those in the Sixth Stage Realm.