For Windows 10 20H2 and 21H1 the ReleaseId for Operating System in General Inventory is wrong and displayed as 2009 :
Client Automation - Any Versions.
If ITCM Agent is in version 14.5, download and install patch SO99111178 on the Agents.
If ITCM Agent is in older version following workaround could be used :
1- Create an Asset Job of type Script and give it a name (ex: Update ReleaseID)
2- In Script tab Copy/Paste the content of attached file Windows_ReleaseID.dms
3- In Scheduling Options, under Miscellaneous tab check the option "This job is allowed to run unattended".