TestUnknownServers.testListUnknownServers is flaky in branch-2

[ERROR] Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 1.913 s <<< FAILURE! - in org.apache.hadoop.hbase.master.TestUnknownServers
[ERROR] org.apache.hadoop.hbase.master.TestUnknownServers.testListUnknownServers  Time elapsed: 0.204 s  <<< FAILURE!
java.lang.AssertionError: expected:<1> but was:<2> 

private List<ServerName> getUnknownServers() {
  if (serverManager != null) {
    final Set<ServerName> serverNames = getAssignmentManager().getRegionStates().getRegionStates()
      .stream().map(RegionState::getServerName).collect(Collectors.toSet());
    final List<ServerName> unknownServerNames = serverNames.stream()
      .filter(sn -> sn != null && serverManager.isServerUnknown(sn)).collect(Collectors.toList());
    return unknownServerNames;
  }
  return null;
}