After KB4338419 is installed, servers running Microsoft Azure AD Connect shows 100% CPU utilization for microsoft.online.reporting.monitoringagent.startup.
We started seeing this in several environments all domain controllers running Windows Server 2012 R2 and Microsoft Azure AD Connect. For our environments KB4338419 was the culprit but it could be any roll-up for .NET Framework 4.6 or 4.7.
At this time it appears to be an Azure AD Connect issue but uninstalling the application isn’t an option so the only immediate resolution is to uninstall the .NET Update. We found uninstalling KB4338419 resolves the issue, at least until Microsoft issues an update to Azure AD Connect.