Cannot reproduce locally. Possibly only failing on wpt.fyi and w3c-test.org?

See https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/resources/testharnessreport.js.

        // Some tests intentionally load mixed content in order to test the
        // referrer policy, so WebKitAllowRunningInsecureContent must be set
        // to true or else the load would be blocked.
        const paths = [
            'service-workers/service-worker/fetch-event-referrer-policy.https.html',
            'beacon/headers/header-referrer-no-referrer-when-downgrade.https.html',
            'beacon/headers/header-referrer-strict-origin-when-cross-origin.https.html',
            'beacon/headers/header-referrer-strict-origin.https.html',
            'beacon/headers/header-referrer-unsafe-url.https.html',
        ];

Test passes locally because of this setting.