The Javadoc says
Enumerates active running StepExecution s in the system.
It is not specified exactly what "active" or "running" mean in this context, but at any rate I have found that StepExecutionIteratorImpl only includes innermost steps, not their containing block steps, because it calls FlowExecution.getCurrentExecutions which is documented to behave that way.
This means that it is impossible to find a StepExecution of a block-scoped step unless it is not currently running its body. (Before or after running the body, it seems to work.) If this behavior is intentional, it needs to be documented more clearly, and I would request some way to get even enclosing executions.