Using classic Outlook for Windows in business environments
Since the groups are visible and fully accessible in Outlook on the web, and other users in your organization can see them in Classic Outlook, this doesn't appear to be an issue with the Microsoft 365 Groups themselves.
To narrow down the cause, could you clarify a few points:
Could you clarify whether the entire Groups section in the Outlook folder pane is missing for the affected users?
What version and build of Classic Outlook are the affected users running? (File > Office Account > About Outlook). Are all affected users on the same Outlook build?
If the steps suggested in Rayyan Fawad’s answer don’t resolve the issue, you may want to try creating a new Windows user profile and testing Outlook there. Since you’ve already recreated the Outlook profile and reinstalled Outlook, this will help determine whether the issue is tied to the Windows profile rather than Outlook itself.
If the issue persists even after those troubleshooting steps, and the Groups remain accessible in Outlook on the web, it may be worth opening a support ticket through the Microsoft 365 Admin Center. Microsoft support can check tenant-level settings and run backend diagnostics that aren’t available in this forum.
For further instructions, you can follow this guide: Get support - Microsoft 365 admin | Microsoft Learn
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