Actually, the deceased Gong Chang and the suspect Meng Yunzheng are both sons of the Gong Family, but they are not Imperial Mandates.
Their case hasn't reached the level that requires the Emperor's attention.
Not to mention that Lord Jing would personally bring the case scrolls to the palace to meet the Emperor after solving the case.
Lord Jing enters the palace naturally to report other matters. The Gong Family case, he will 'incidentally' inform the Emperor, as this issue involves not only the Gong Family but also Duke Liu.
Presumably, the Emperor will be interested in listening to the entire story.
Sure enough, after Lord Jing finished the important affairs, he casually presented the scrolls of the recently concluded case for the Emperor to review, as if inadvertently mentioning the Gong Family's murder case.
