20H2 Enterprise downgrades itself to Pro after removing from the domain and then running sysprep

Question

Hi everyone, this may not be explicitly an MDT issue, but I use MDT, and I'm not entirely sure where the issue is.

Symptoms -

• You deploy domain-joined 20H2 Enterprise straight from VLSC ISO (OR by deploying an existing 1909 image, upgrading to 20H2 using Windows Upgrade Assistant).
• You remove it from the domain, and Capture the image.
• You deploy the image, but the version has been changed from Enterprise from Pro (and KMS no longer works).

Apparent Cause -

• I have determined that the apparent cause is at the point of removing from the domain on 20H2 the first time, the BackupProductKeyDefault registry entry is for some reason replaced from the default backup Enterprise value to the default backup Pro value (this did not happen with 1709, 1809, 1909). However this only becomes problematic after sysprep.

Result -

• The main Enterprise key stays in place seemingly until Sysprep. However during Sysprep, Windows appears to fall back initially on this backup key value, and effectively downgrades Windows to Pro and breaks KMS in the process (thus breaking your MDT image effectively).

Workaround -

• AFTER removing from the domain, check if the key has indeed changed, and if it has re-Activate Windows using the default public generic KMS Key (NPPR9-FWDCX-D2C8J-H872K-2YT43) - this in turn also overwrites the BackUpProductKeyDefault with the correct value, and possibly corrects some other critical values.
• This seems to be a permanent fix from what I can tell, as the image can now be happily re-added and removed, deployed, and removed, etc without this key being incorrectly replaced again.

The Questions -

I'm pleased to have found a workaround and hope it helps someone else. But I'd like to understand why this is happening? Why did it never happen until 20H2? Is it a bug in the code of the 20H2 installer, changing to the wrong version?

Cheers all.

Ben.