When a subprocess ends in STATE_ABORTED due to a error event being throwed with FaultName / FaultVariable parameters defined and set, the boundary event in the parent process receives the FaultVariable data set to null.

The reason seems to be in line 238 of org.jbpm.workflow.instance.node.SubProcessNodeInstance method processInstanceCompleted:

exceptionScopeInstance.handleException(faultName, null);

Instead of that "null", the FaultVariable information should be provided.