Assign Message with multiple variables using Assign Variable

[sethdickerson]

This proxy is extracting 2 variables in a proxy request flow. After the variables have been confirmed as extracted (via trace logs), I am using an Assign Message to format a request to be sent to the back end target server.

What I am doing:

If the client provides the parameter in the request, then I will extract the variable, and assign it as "variable.A".

If the client does not provide the parameter in the request, then I will assign it a defaulted variable obtained from an Access Entity policy triggered in the proxy preflow and call it "variable.B".

The AssignMessage/AssignVariable should determine:

- If "variable.A" does not exist or is null, then use "variable.B"

Below is the AssignMessage:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage async="false" continueOnError="false" enabled="true" name="AM-productPricingPayloadDefault">
    <AssignVariable>
        <Name>productId</Name>
        <Ref>variable.A</Ref>
        <Template>{variable.B}</Template>
    <AssignVariable>

What I am seeing:

If "variable.A" is provided by the client, then the proxy executes as expected. However, if "variable.A" is not provided, then the proxy does not substitute using "variable.B".

What I expect to see:

If the client does not send parameter to populate "variable.A", then the AssignVariable would look to the "Template" section and use "variable.B".

Again, I have confirmed that "variable.B" is a legitimate/populated cached variable via Apigee Trace Logs and according to the Apigee Docs (https://docs.apigee.com/api-platform/reference/policies/assign-message-policy#assignvariable). What I am attempting should work using the Template functionality so I am assuming I simply have a format/syntax issue that I am not seeing. I would also prefer to NOT use a JavaScript for this. Any assistance would be appreciated.