Symptoms
The MonitoringGroup settings can't be used reliably to make sure the CAS monitoring mailboxes can be created as expected if the organization has more than 11 DAGs in Microsoft Exchange Server 2019 or Exchange Server 2016.
Resolution
To fix this issue, install one of the following updates:
For Exchange Server 2019, install the Cumulative Update 9 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.
For Exchange Server 2016, install the Cumulative Update 20 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Note: After installing the update, you need to run the following cmdlet to enable the feature:
Add-GlobalMonitoringOverride -Identity Monitoring\Configuration -ItemType Maintenance -PropertyName EnableIterateAllDags -PropertyValue true
References
Learn about the terminology that Microsoft uses to describe software updates.