Github user GJL commented on the issue:
https://github.com/apache/flink/pull/4980
There is only one thread dispatching the calls:
```
executor = Executors.newSingleThreadExecutor(
new
DispatcherThreadFactory(TASK_THREADS_GROUP, "Async calls on " +
taskNameWithSubtask));
this.asyncCallDispatcher = executor;
```
The tasks cannot overtake each other. I could make the test more strict and
wait additionally on `triggerLatch` in case somebody decides to have multiple
threads.---
