I'm following the SagePay API Docs to integrate 3D Secure: <https://developer.sage.com/api/payments/additional-integrations/3-D-secure/>

In TEST, when I don't send secureCustomerAuthentication object I get sent down the fallback route which is fine. However, when I do send the secureCustomerAuthentication object all transactions are coming back as 'Authorised' and it doesn't invoke the 3DS/cRes flow.

I've tried every card listed here:
<https://www.opayo.co.uk/support/12/36/test-card-details-for-your-test-transactions>

Any ideas how I could invoke the SagePay 3DSv2 flow?