Windows node seems buffering the last emit

I might have missed a setter or some configuration options.

|window()
.period(1s)
.every(1s)
|log()

If I send 10 messages, 1s apart. The last message will not show up in log node output, no matter how long I waited. I have to push another message to make 10th message appear, but again the 11th message will not show up.

I have tested with align(), fillPeriod() setters. None of them makes difference.

We are using Kapacitor 1.3.1.