Unable to @mention guest users in a Word document

This is a known limitation of the commenting feature in SharePoint for Office documents and Microsoft Lists.

Currently, you cannot @mention external or guest users in MS Office documents or in Microsoft Lists comments.

Please refer to the article below for more information:

SharePoint Online/Microsoft Lists: @mention people in list comments 

Unable to @ mention external collaborators in comments within Office online files | Microsoft Commu…

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