We've identified a problem in the New Response Workflow Rules. If a system has an active New Response Alert configured to open a case without assigning it, and this alert has no recipients (meaning it only opens the case), the New Response Workflow Rules would not be triggered for that particular system.
This issue occurred specifically between 12:00 and 16:00 CEST time. The issue has been resolved, and the backlog has been reprocessed.