EventTimeSessionWindow firing too soon

[Ori Popowski]

I'm using Flink 1.10 on YARN, and I have a EventTimeSessionWindow with a
gap of 30 minutes.

But as soon as I start the job, events are written to the sink (I can see
them in S3) even though 30 minutes have not passed.

This is my job:

val stream = senv
      .addSource(new FlinkKafkaConsumer("…", compressedEventDeserializer,
properties))
      .filter(_.sessionId.nonEmpty)
      .flatMap(_ match { case (_, events) => events })
      .assignTimestampsAndWatermarks(new
TimestampExtractor[Event](Time.minutes(10)) {
        override def extractTimestamp(element: Event): Long =
event.sequence / 1000 // microseconds
      })
      .keyBy(_.sessionId)
      .window(EventTimeSessionWindows.withGap(Time.of(30, MINUTES)))
      .process(myProcessWindowFunction)

AsyncDataStream.unorderedWait(stream, myAsyncS3Writer, 30, SECONDS, 100)

Any idea why it's happening?