That would be because that site is hosted on a Google service, but depends on the hostname to figure out which customer/backend to use. This is normal for any m

That would be because that site is hosted on a Google service, but depends on the hostname to figure out which customer/backend to use. This is normal for any multi-tenant hosting. Your CNAME is not registered for that application in the hosting service's database and therefore they can't tell what it's for. Remember that a CNAME isn't a redirect or anything, there's literally no way for the receiving webserver to know the domain that was CNAME'd in these sorts of requests.