JbpmVariableResolver first checks if the current token has a variable and then retrieves the given variable from root token (lines ##36-38):
if (contextInstance.hasVariable(name, token))

{ //succeeds return contextInstance.getVariable(name); //returns NULL }

It should be (compare to the jbpm-jpdl v. 3.2.8):
if (contextInstance.hasVariable(name, token))

{ return contextInstance.getVariable(name, token); }

since the current token is not always the root.