Autopilot profile is not assigned if a device already registered Azure AD

When import device information for Autopilot, if the devices already registered to Azure AD, the profile status in Windows Autopilot devices have not changed from ”Not Assigned”.

After deleting the device from both Autopilot devices and Azure AD, and import again, it has changed to ”Assigned”.

It is the same behavior at import csv file, which created by PowerShell script manually or use Autopilot profile to convert targeted devices.

Note that we had not applied Autopilot enrollment to all devices, and we use a security group referring ”ZTDId”. And a security group specified Azure AD devices for convert.

And, it seems when import csv file to Autopilot, devices register to Azure AD automatically. If we remove it from Azure AD, the device information will not re-register automatically. Or will it work with the security group of all Azure AD device?

I want to know:
1. Import Autopilot information of devices that already registered to Azure AD does it work?
2. With the security group reference ZTDId, does it work?
3. The best practice to register Autopilot information, that device already registered to Azure AD and Intune

Thank you for reading. If you know anything about it, please teach me.