Dear Robhelp,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, it seems that you are having issue with access management in Loop, cannot remove the direct access assigned with @ mention.
If the understanding above is right, we've done a quick test, and we can reproduce a similar situation on our side.
Considering that issue can be reproduced with different users and different tenants, we're afraid that issue could be related to the background service which need to be checked from the backend. As forum support, we may not have enough permission and resource to directly check the backend environment.
In this situation, we sincerely recommend you contact your Microsoft 365 administrator, follow steps in this article to create a support ticket with Online Support. The support team over there has higher permission than us, they can remotely help check the situation on your side, involve more resource to collect logs and help you do troubleshoot. This would be a more efficient way in handling this case for you.
For reference: How do I find my Microsoft 365 admin?
Sorry for the inconvenience again. Your patience and understanding are highly appreciated. Hope you stay safe and healthy.
Sincerely,
Rhoda | Microsoft Community Moderator