The Activity stream @ mention functionality on a form suppose to trigger an email to the user who is mentioned, but the email was not triggered on an affected instance.

• "Activity Stream @Mention Email" notification record was not active on the affected instance (You can verify this via "System Notification=>Email=>Notoifications and filter for Name = Activity Stream @Mention Email").
• Thus users couldn't set the "Activity Stream @Mention Email" notification preference to true.
• Hence, @ mention emails are not created and sent

• Please activate the "Activity Stream @Mention Email" notification record on the affected instance and then set this user preference true for a user, this should resolve the issue.
• Also, make sure the email field is set on sys_user record, thus it will create cmn_notif_device record to send an email notification for @mention functionality.