What is the reason for the jussive mood of the verb "nabgh" in the noble verse "That is what we were seeking" in Surat Al-Kahf?

The recitation of (ذلك ما كنا نبغِ) [That is what we were seeking] has been narrated with several readings: Abu Amr, Nafi', and Al-Kisa'i recited it by connecting the "ya" and pausing by omitting it. Ibn Kathir, however, recited it with the "ya" connected both in continuity and pause. As for Asim, Ibn Amir, and Hamza, they recited it by omitting the "ya" both in continuity and pause. The omission of the "ya" for lightness is present in the Quran, such as (والليل إذا يسرِ) [And the night when it departs] and (فيقول رب أهاننِ) [Then he says, "My Lord has humiliated me"].