Pre-Stage Imaging showing computers as "Unmanaged"

I did not configure Management account because we are using management account with random password.

Can you explain this? Are you saying your config doesn't contain any management account configured at all?

@mm2270, yes Management Account details were not provided.

Well, then that would appear to be the issue. A Mac can't be "Managed" if the record doesn't have a valid local admin account credentials for the management account. It either must be created at the time of enrollment, or you need to plug in something that already exists on the device. It's just part of how Jamf manages the Macs.
I get that you want to randomize the password for that management account, but the approach you should take is to initially create a basic management account with a static password that the device will get set up with, then have a one time (or ongoing) policy targeted perhaps to new enrollments, that will randomize the password. This allows the jamf binary to randomize it, and update the computer record back in the Jamf Pro db with the new credentials. You don't need to know what the password is, but the database must know or it can't manage the Mac.

@mm2270, Thank you so much for your detailed explanation, I will test this now...

I have a question here, If we are imaging the computer using pre-stage imaging the computer will be enrolled automatically. But in my case enrollment is not happening at all, do i need to enroll explicitly? Please correct me If I am wrong

Thanks Once again for your help