Hey Ian,
This seems like it would be perfectly suited as a defect report at the
Public Issue Tracker for the Cloud Platform. If you provide all relevant information, along with your project ID, the domain name, and a
HAR recording of your attempted verification, we'll quickly reply and get to work on determining the root cause of the issue. We monitor the Public Issue Trackers every day, so rest assured you'll see a response. This thread here is slightly off-topic since the App Engine Google Groups forum is meant more for general discussion of the platform and services.
Best wishes,
Nick
Cloud Platform Community Support