SetWorkflowVariable does not set the value

I have a the_var Wotkflow Variable. In the automation chain of the transition of a "Step1" task, I set up this variable using a simple chain:

- Context.FetchDocument
- Context.SetWorkflowVar:
    name: the_var
    value: "hello there!"

Now, in the "inbound" automation step of Step#2, I just check this variable:

- Context.FetchDocument
- Log:
    level: warn
    message: "Value of the_var: @{WorkflowVariables[\"the_var\"]}"

The value is null

My original use case is more complex and problematic. The Workflow Variable is claimRep. In the transition, it is set using the CurrentUser, and the next task has a "dynamic assignee" basic expression...

{{WorkflowVariables["claimRep"]}}

This triggers errors, since...

{{WorkflowVariables["claimRep"]}}

...returns null => Nuxeo can't assigns the task and breaks.