Netsparker identified that Expect-CT is in report only mode. The optional enforce directive controls whether the browser should drop the connection when the policy is violated.
When Expect-CT policy is deployed in report only mode and the user agent does not receive a valid Certificate Transparency Log, rather than dropping the connection it will simply send a report to the specified endpoint which is set with report-uri directive.
Use enforce flag in definition of Expect-CT.
Expect-CT: enforce, max-age=7776000, report-uri="https://ABSOLUTE_REPORT_URL"