Can test AXP Offer Callback task in a lab environment?

I'm trying to validate the callback mechanism in a self-service flow. The issue is that the Offer Callback? task always takes the No path.

Test setup:
Administration > Contact Center > Callback > minimumEWT=1 maximimEWT=480
My queue business hours is set to None (24 hours).
My Orchestration flow goes to the "OneCloud Callback Offer Self Service" flow.

The self-service flow always takes the No path at the "Offer Callack?" step.

In my test lab I have a single agent ready in the queue. When I call in the first time, the caller is connected to the agent as expected. I leave that call active for a few minutes while I call the number on another line. This 2nd caller still sees the EWT variable as the default value, the Offer task takes the No path, and the 2nd caller is waiting for the agent to be done with the prior call.

I can repeat the call with the same result but I was expecting the new caller to be offered a callback as they are waiting for the agent to become available.
How can I set up a callback test in my lab with a single agent? Possible?
Is there any way to diagnose the problem of why the EWT is not being updated?

I saw in the drop45 release notes that it calculates the EWT by: "invoking the synchronous Query Metrics API". Is this an API that a developer needs to directly access to make callbacks work?