AFAIK, the map you inserted belongs to the ProcessInstance "testflow-web". Hence to get the data from the map, you have to obtain the ProcessInstance inside your work item handler then use the getVariable() method to obtain the parameter you passed in when you started the process.
The parameter mapping used in the nodes in the diagram is for mapping variables which are present in the diagram itself (Open the bpmn diagram's properties and there is a row for variables where you can add/remove variables used in the Parameter Mapping