icloud.com doesn't load in M66. Why?

If we go to the console we see:

"The SSL certificate used to load resources from https://feedbackws.icloud.com will be distrusted in M70. Once distrusted, users will be prevented from loading these resources. See https://g.co/chrome/symantecpkicerts for more information."

Right, but it's not M70, it's M66. If we go directly to the URL that seems to be failing, https://appleid.cdn-apple.com/appleauth/static/jsj/1342917900/widget/auth/app.js , we get a generic NET::ERR_CERT_AUTHORITY_INVALID interstitial.

However, if we try to load that URL in M65, we see:

"The SSL certificate used to load resources from https://appleid.cdn-apple.com will be distrusted in M66. Once distrusted, users will be prevented from loading these resources. See https://g.co/chrome/symantecpkicerts for more information."

If we hit a Symantec cert in M66, we should log saying, "hey, the reason for NET::ERR_CERT_AUTHORITY_INVALID is the Symantec cert". Otherwise it's not clear what's happening, and it's not obvious that old Chromes would give a better diagnostic here.