"It's planted."
At the start of Su Chen and Yang Qi's fierce battle, Hongying did not flee. She wanted to see Su Chen die right before her eyes.
Unfortunately, as the battle progressed, Yang Qi was completely suppressed by Su Chen. He had no ability to strike back, growing increasingly pathetic to the point where it seemed death was a possibility.
Hongying was genuinely frightened.
She no longer had the courage to keep watching and chose to flee outright.
When she fled, Su Chen did not stop her.
There were two reasons.
First, he was locked in a fierce battle with Yang Qi, unable to afford any distraction. Even the slightest loss of focus could decide the outcome of the fight.