Flink portable runner gets stuck when waiting for SDK Harness to close

When parallelism = nproc:

"MapPartition (MapPartition at [37]

) (9/12)" #2855 prio=5 os_prio=0 tid=0x00007f9184022800 nid=0x2b58 waiting on condition [0x00007f9091592000]
java.lang.Thread.State: WAITING (parking)
at (C/C++) 0x00007f926a97a9f2 (Unknown Source)
at (C/C++) 0x00007f9269f1dd99 (Unknown Source)
at sun.misc.Unsafe.park(Native Method)

• parking to wait for <0x00000000ca218030> (a java.util.concurrent.CompletableFuture$Signaller)
  at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
  at java.util.concurrent.CompletableFuture$Signaller.block(CompletableFuture.java:1693)
  at java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3323)
  at java.util.concurrent.CompletableFuture.waitingGet(CompletableFuture.java:1729)
  at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1895)
  at org.apache.beam.sdk.fn.data.CompletableFutureInboundDataClient.awaitCompletion(CompletableFutureInboundDataClient.java:48)
  at org.apache.beam.sdk.fn.data.BeamFnDataInboundObserver.awaitCompletion(BeamFnDataInboundObserver.java:90)
  at org.apache.beam.runners.fnexecution.control.SdkHarnessClient$ActiveBundle.close(SdkHarnessClient.java:298)
  at org.apache.beam.runners.flink.translation.functions.FlinkExecutableStageFunction.$closeResource(FlinkExecutableStageFunction.java:209)
  at org.apache.beam.runners.flink.translation.functions.FlinkExecutableStageFunction.mapPartition(FlinkExecutableStageFunction.java:209)
  at org.apache.flink.runtime.operators.MapPartitionDriver.run(MapPartitionDriver.java:103)
  at org.apache.flink.runtime.operators.BatchTask.run(BatchTask.java:503)
  at org.apache.flink.runtime.operators.BatchTask.invoke(BatchTask.java:368)
  at org.apache.flink.runtime.taskmanager.Task.run(Task.java:712)
  at java.lang.Thread.run(Thread.java:748)