It appears that you are experiencing a significant issue with the rendering of .docx icons in File Explorer, which may indeed indicate a problem with the shell integration of Office Professional 2021. Given that you have already performed extensive troubleshooting steps, including verifying registry keys, rebuilding the icon cache, and reinstalling Office, it seems that the issue is more complex than a typical user error.
Unfortunately, the context provided does not contain specific information regarding known issues with Office shell integration or escalation procedures for support. However, it is suggested that you provide feedback directly to Microsoft through the Help > Feedback > Make a suggestion option in Office applications, as this can help bring attention to the issue.
For further assistance, you may want to reach out to Microsoft Support directly to escalate the issue to a senior support engineer or the product team responsible for Office shell extensions. They would be in a better position to confirm if this issue is known and if a fix or workaround is available.
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