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Dear DRS,
Good day!!
We appreciate for sharing the workaround for the concern facing when using @mention in the Comments of the SharePoint Online list and where we believe it will help the users in this community who are having similar issue.
Moreover, I have done the quick test in the SharePoint Online list by @mentioning the user who is not having access to the SharePoint Online list and I didn’t encounter any problem as explained in the reference thread.
So, if you are facing the same issue as the reference thread i.e., permission error when @mentioning any users at the Comments section of item in the List, I believe the problem should be further investigated by related development team via collecting HAR logs because there is no any related service incident reported at the Microsoft Service Health dashboard.
I do feel regret for not resolving the problem and redirecting you to the related development team but the fix to your concern can be find out through the HAR logs and suggest you raise a support ticket from the Office 365 admin center, where the related team will investigate the problem further by collecting the HAR logs.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: To create a support ticket, you need to have admin privileges and if you aren’t admin of your Office 365 organization, please contact the admin of creating a new ticket
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia