beam_PostCommit_Java_ValidatesRunner_Dataflow_* failure

Failing test:
https://ci-beam.apache.org/job/beam_PostCommit_Java_ValidatesRunner_Dataflow_Java11/3247/
org.apache.beam.sdk.transforms.ParDoTest$TimerTests.testEventTimeTimerLoop

 
```

java.lang.RuntimeException: java.lang.RuntimeException: org.apache.beam.sdk.util.UserCodeException:
java.lang.AssertionError: ParDo(Anonymous)/ParMultiDo(Anonymous).output:
Expected: iterable with items
[<0>, <1>, <2>, <3>, <4>] in any order
but: not matched: <0>
at org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowsParDoFn$1.output(GroupAlsoByWindowsParDoFn.java:187)
at
org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowFnRunner$1.outputWindowedValue(GroupAlsoByWindowFnRunner.java:108)
at
org.apache.beam.runners.dataflow.worker.util.BatchGroupAlsoByWindowViaIteratorsFn.processElement(BatchGroupAlsoByWindowViaIteratorsFn.java:128)
at
org.apache.beam.runners.dataflow.worker.util.BatchGroupAlsoByWindowViaIteratorsFn.processElement(BatchGroupAlsoByWindowViaIteratorsFn.java:56)
at
org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowFnRunner.invokeProcessElement(GroupAlsoByWindowFnRunner.java:121)
at
org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowFnRunner.processElement(GroupAlsoByWindowFnRunner.java:73)
at
org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowsParDoFn.processElement(GroupAlsoByWindowsParDoFn.java:117)
at
org.apache.beam.runners.dataflow.worker.util.common.worker.ParDoOperation.process(ParDoOperation.java:44)
at
org.apache.beam.runners.dataflow.worker.util.common.worker.OutputReceiver.process(OutputReceiver.java:49)
at
org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation.runReadLoop(ReadOperation.java:212)
at
org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation.start(ReadOperation.java:163)
at
org.apache.beam.runners.dataflow.worker.util.common.worker.MapTaskExecutor.execute(MapTaskExecutor.java:83)
at
org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.executeWork(BatchDataflowWorker.java:420)
at
org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.doWork(BatchDataflowWorker.java:389)
at
org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.getAndPerformWork(BatchDataflowWorker.java:314)
at
org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.doWork(DataflowBatchWorkerHarness.java:140)
at
org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.call(DataflowBatchWorkerHarness.java:120)
at
org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.call(DataflowBatchWorkerHarness.java:107)
at
java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

```

Recent changes to timer in this pr: https://github.com/apache/beam/pull/16650

Imported from Jira [BEAM-13833](https://issues.apache.org/jira/browse/BEAM-13833). Original Jira may contain additional context.
Reported by: kileysok.