It appears that calling queue.wait() after the graph has been submitted causes incorrect behaviour/results in the example code, like so:

  // Execute several iterations of the graph
  for (unsigned n = 0; n < iterations; n++) {
    testQueue.submit([&](handler &h) { h.exec_graph(graphExec); });
  }
  // Perform a wait on all graph submissions.
  testQueue.wait();

In reality this should at the very least do nothing if the graph has already been executed (as happens in the current prototype).