Additional team and channel-related features and issues within Microsoft Teams for business
Based on the testing described, this does not look like a user-specific Teams notification setting issue.
The important distinction is that the @mention is still being generated and delivered by email, while the corresponding Teams Activity feed notification is missing for all users. Microsoft documents that @mentions in comments can be surfaced through the Teams activity feed, so the behavior described is consistent with a failure somewhere in the Lists/SharePoint-to-Teams notification path rather than the underlying mention itself.
I would recommend checking the following:
Confirm the behavior in Teams desktop and Teams on the web. This helps determine whether the issue is client-specific.
Check Teams > Settings > Notifications and activity and verify that app/activity notifications have not been changed.
Test a newly created list and a newly created comment/@mention to determine whether the issue is specific to existing lists.
Check Microsoft 365 Service Health for any active incident affecting SharePoint, Microsoft Lists, or Teams notifications.
Record the exact UTC time of several failed @mentions, affected users, list/site URL, Teams team/channel, and whether the email notification was generated.
Because the issue affects multiple users and multiple Lists locations while other Teams @mentions continue to work, I would escalate this to Microsoft support with the above timestamps and examples and specifically request investigation of the service-side notification/Activity feed delivery path.
I would not recommend recreating the Teams team or changing tenant-wide notification policies as a first step. The evidence provided does not point to a general Teams @mention failure.
Also, avoid posting tenant IDs, user IDs, private list URLs, or other identifying information publicly. Provide those details only through the Microsoft support case.