title: Flatten fails to return any data in CoGroupByKey
body: <p>I am currently using apache beam 2.4. My pipeline includes a CoGroupByKey step which does not return any data.
  As I expand the task in the UI, the data does not flow beyond the internal Flatten.PCollection of CoGroupByKey.</p>
  <p>My code is structured as presented in the <a href="https://beam.apache.org/documentation/sdks/javadoc/2.4.0/index.html?org/apache/beam/sdk/transforms/join/CoGroupByKey.html" rel="nofollow noreferrer">javadoc</a>  example</p>
  <pre><code>static final TupleTag&lt;String&gt; simpleEventTag1 = new TupleTag&lt;String&gt;(){};
  static final TupleTag&lt;String&gt; simpleEventTag2 = new TupleTag&lt;String&gt;(){};
  PCollection&lt;KV&lt;String, String&gt;&gt; pt1 = ...;
  PCollection&lt;KV&lt;String, String&gt;&gt; pt2 = ...;
  
   PCollection&lt;KV&lt;String, CoGbkResult&gt;&gt; coGbkResultCollection =
     KeyedPCollectionTuple.of(simpleEventTag1, pt1)
                          .and(simpleEventTag2, pt2)
                          .apply(CoGroupByKey.create());
  </code></pre>
  <p>My window is:</p>
  <pre><code>Window.&lt;String&gt;into(FixedWindows.of(Duration.standardMinutes(1)))
      .triggering(Repeatedly.forever(AfterProcessingTime
      .pastFirstElementInPane()
      .plusDelayOf(Duration.standardSeconds(20))
      )
  )
  .withAllowedLateness(Duration.ZERO)
  .discardingFiredPanes()
  </code></pre>
  <p>pt1 and pt2 are both retrieved from a PCollectionTuple and are not empty. They share the same FixedWindow properties.
  I don't know what I am doing wrong and could not find any way to debug this issue. Any idea ?</p>
  <p><a href="https://i.stack.imgur.com/byIq7.png" rel="nofollow noreferrer">Expanded CoGroupBy</a></p>