RxJS groupBy and combineAll operators seem to omit output

When grouping output with the combination of .groupBy and .concatAll, some expected output is not generated.

Sample code:

var Rx = require('rx');

var source = Rx.Observable.from(['a1', 'a2', 'b1', 'b2', 'a3', 'a4', 'b3', 'b4'])
  .groupBy(function (item) { return item.substr(0, 1); })
  .concatAll();

var subscription = source.subscribe(
  function (x) {
    console.log('Next: %s', x);
  },
  function (err) {
    console.log('Error: %s', err);
  },
  function () {
    console.log('Completed');
  });

Actual output:

$ node index.js
Next: a1
Next: a2
Next: a3
Next: a4
Completed

Expected output:

$ node index.js
Next: a1
Next: a2
Next: a3
Next: a4
Next: b1
Next: b2
Next: b3
Next: b4
Completed

Am I misunderstanding how these operators work? Or is this an RxJS bug? (Tentatively filed as a bug already at https://github.com/Reactive-Extensions/RxJS/issues/1264.)