Murong Jiu knew about Consort Jing's past as the Emperor's beloved, but it was pieced together from a few complaints of the Second Prince.
She had never seen Consort Jing.
By the time she learned about this, Consort Jing had been caught in bed with a guard for "adultery" and then hung herself.
Meanwhile, The Third Prince also drowned in a river on his way back to the Capital.
Both the framing and assassination were done by the Second Prince and the Empress.
The Emperor, even if he wanted to deal with it, couldn't find any evidence, let alone the fact that he was probably under the influence of a slow-acting poison at the time, often bedridden.
Before Murong Jiu died, the Second Prince had already been assisting the Emperor with court affairs.
The sudden sight of Consort Jing left Murong Jiu genuinely unable to recognize her.