No AfterWatermark firings in Dataflow

[Aaron Dixon]

I have the following trigger:

.apply(Window
      .configure()
      .triggering(AfterWatermark
           .pastEndOfWindow()
           .withEarlyFirings(AfterPane
                .elementCountAtLeast(1)))
      .accumulatingFiredPanes()
      .withAllowedLateness(Duration.ZERO)

But in Dataflow I notice that I never get an ON_TIME firing for my window
-- I only see early firing for elements, and then nothing.

My assumption is that AfterWatermark should give me a last, on-time pane
under this configuration when the watermark surpasses the window's end.

Is my expectation correct?