We've confirmed that all systems are back to normal with no customer impact as of 5/18, 18:34 UTC. Our logs show the incident started on 5/18, 17:35 UTC and that during the 59 minutes that it took to resolve the issue some of customers may have experienced issues with delayed alerts in East US2 and Central US regions.
Root Cause: The failure was due to an issue in one of our dependent services.
Incident Timeline: 59 minutes - 5/18, 17:35 UTC through 5/18, 18:34 UTC
We understand that customers rely on Azure Monitor Alerts as a critical service and apologize for any impact this incident caused.