Thank you for your post into the forum.
The SQL Server Agent not running alert is generated when either the SQL Server Agent service is in a stopped state or encountered a problem which has caused it to fail.
The SQL Server Agent not running alert (along with the Job Failure, Job did not start and Job Duration unusual alerts) is checked every 60 seconds.
The computer unreachable and SQL Server unreachable alerts are checked every 5 mins, which may explain why you do not experience these particular alerts
A support call has been created for you, the support call reference is F0033005. Can you please sent to us a copy of the Alert Repository Log File you us to analyze via e-mail to firstname.lastname@example.org
with the call reference number in the subject field of the e-mail.