This generally happens when the resource in ConfigMgr doesn't have an AAD ID populated.
The log for this AAD group sync, starting in 2002, is CollectionAADGroupSyncWorker.log.
i have an SCCM collection synched with Azure AD group .. just some of the machines are not being synched to the Azure AD group while they exist in SCCM collection .
any idea where to start troubleshooting ? all logs on Site server seems ok ( SMS_AZUREAD_DISCOVERY_AGENT.log) .