We're using the analytics client in our tests as so:

// before test
val analytics: Analytics = Analytics
  .builder(writeKey.value)
  .endpoint(wireMockUrl.toASCIIString())
  .build()

// during test
val message = TrackMessage
      .builder("my-event-name")

analytics.enqueue(message)

// before asserting anything
analytics.flush()
analytics.shutdown()

with a mock HTTP client that just gathers all requests.

We are seeing our tests be flaky because sometimes that message never hits the mock server.

Looking at AnalyticsClient.java I'm pretty sure it's possible for the call to AnalyticsClient.shutdown() to shutdown the networkExecutor before the looper has submitted the last batch to it; you never interrogate the Future<?> returned when you call looperExecutor.submit(new Looper()) so the RejectedExecutionException thrown when Looper calls networkExecutor.submit(BatchUploadTask) after the networkExecutor has been shutdown is silently swallowed.