InvalidJsonReferenceFormat - unexpected Reference ID for subnets

[cisco-csr-1000v](https://github.com/Azure/azure-quickstart-templates/tree/master/cisco-csr-1000v)
### Issue Details

Deployment fails to target subnets.

Error:
{
"error": {
"code": "InvalidRequestFormat",
"message": "Cannot parse the request.",
"details": [
{
"code": "InvalidJsonReferenceFormat",
"message": "Reference Id /subscriptions/48db2aff-235a-4d2d-927c-8ddd511aefb3/resourceGroups/existing/providers/Microsoft.Network/virtualNetworks/Basic-Infrastructure/default is not formatted correctly. The Id is expected to reference resources of type virtualNetworks/subnets. Path properties.ipConfigurations[0].properties.subnet."
}
]
}
}

Start with the cisco-csr-1000v template linked in the issue and inspect the subnet reference used by properties.ipConfigurations[0].properties.subnet. Reproduce the deployment failure and verify that the request references the expected virtualNetworks/subnets resource rather than the shown virtualNetworks/default path.

Written by the indexing model from the issue text.