Is the subject line for interactions in Genesys Cloud for Salesforce and Genesys Cloud for Zendesk always automatically populated?
Usually. The integrations create subject lines for interactions when the interactions connect. However, if an outbound call disconnects before the external party picks up, the activity record (in Salesforce) would not contain a subject.
For more information about the integrations, see About Genesys Cloud for Salesforce and About Genesys Cloud for Zendesk.