Hi,
Thank you for your sharing.
Just be sure target OS build > existing OS build would not cause the issue. So no matter uninstall 9C or 10B update package on existing OS or add them on target OS are works well.
The known issue has been released by Microsoft in Windows Update history on 9C and 10B package link no matter 1909, 2004 or 20H2. Here is a link for quick action. https://support.microsoft.com/en-us/help/4555932
And also has been states in Windows 10, version 20H2 and Windows Server, version 20H2
If any information is useful for you, please accept it as answer.
Bests,