Exalted be Allaah! In spite of all this material, spiritual, diplomatic and economic preparation, and the motivation undertaken by the scholars to urge people to fight, some men with weak souls did not believe that the fight with the Tatars had become a fait accompli. Of course, I mean those with weak hearts rather than weak bodies, who, during the preparation of the army, thought it to be no more than enthusiastic glamour which would soon be extinguished.

They thought that those were but words which Qutuz, May Allaah have mercy upon him, said, like other leaders, to deceive their people, and give vent to pressure, in avoidance of explosion. They never thought that Qutuz, May Allaah have mercy upon him, was really preparing for the fight. However, when the zero hour drew near, they became certain that it was really a fact and that the confrontation would be very close.

At that point, the hearts of those men, or say, the puppet men, shook and they started to think about fleeing from the army and disappearing from the sight of the observers; and some of them actually escaped from Egypt to other countries like Hijaaz, Yemen and even the Arab Maghrib.
Some analysts may think it was a loss for the army which seemed to have lost some of its important elements, or at least became few in number in the sight of the enemies.

Exalted be Allaah! Good, all good, lies in the flight of those in the moments prior to fighting. It may be that Allaah The Almighty Would Make difficult the moments prior to fighting, in order to Purify the row of fighters, and no one should set out for the fight save the one who really intended to remain steadfast.
What would happen if those hesitant came out among the Muslim army?
Allaah The Almighty Gave the reply to that question in the Surah of At-Tawbah Saying (what means): {Had they gone forth with you, they would not have increased you except in confusion, and they would have been active among you, seeking [to cause] you fitnah. And among you are avid listeners to them. And Allaah Is Knowing of the wrongdoers.} [Quran 9:47]

Had those who were hesitant set out with the Muslim army, they would have weakened its power and spread trouble and worry among its people, once unintentionally by their fear and terror, and once intentionally to provoke Fitnah and break the unity of the row. However, the greater problem lies in the fact that some truthful Muslims might listen to their words and be convinced of their doubts, thereupon the Muslims would lose further power, given that, as stated by Allaah The Almighty (what means): {among you are avid listeners to them.} For this reason, the flight of those at such sensitive moments was good for the benefit of the Muslim army, no matter how different it seemed to the eyes of the people at that time. That is why Qutuz, May Allaah have mercy upon him, was not eager to rejoin those once again, since the real military in Islam should be longed for rather than escaped from.
In this way, the Muslim army was purified.

https://en.islamstory.com/en/artical/20797/muslim-army-purified