When you configure Message Box Data Monitoring for Terminating Suspended Service Instances, you may see this Failure or Warning report in the Data Monitoring Dashboard, as well as a failure alert in an email.
The Overall status will be shown as failure or warning due to the Message Box Data monitoring configuration, based on "Set the Notification condition". If you have set the condition that when it has reached above "20", an Error alert should be triggered. That's the reason for receiving the "Failure alert", but the Action taken is success.
Overall Status - Displays either Success, Failure (Error), or Warning depending on the status of data monitoring execution.
For more details, you can find the articles below.
To understand Message Box Data Monitoring Email Notifications,
https://docs.biztalk360.com/v12/docs/message-box-data-monitoring#data-monitoring-execution-results