cloud-spanner-emulator: Cannot insert multiple rows in one transaction with a commit timestamp column

I have a table with a TIMESTAMP column with OPTIONS (allow_commit_timestamp=true). If I try to execute two INSERT statements on this table with a PENDING_COMMIT_TIMESTAMP() value for the commit timestamp column, the second statement will fail with a INVALID_ARGUMENT: Column LastModified cannot be accessed because it, or its associated index, has a pending CommitTimestamp error message.

Example code that reproduces the problem:

  public static void main(String[] args) throws Exception {
    try (Spanner spanner = SpannerOptions.newBuilder().setProjectId("test-project")
        .build().getService()) {
      try {
        spanner.getInstanceAdminClient().createInstance(
            InstanceInfo.newBuilder(InstanceId.of("test-project", "test-instance"))
            .setDisplayName("test")
            .setNodeCount(1)
            .setInstanceConfigId(InstanceConfigId.of("test-project", "emulator-config"))
            .build()
            ).get();
      } catch (ExecutionException ee) {
        SpannerException e = (SpannerException) ee.getCause();
        if (e.getErrorCode() != ErrorCode.ALREADY_EXISTS) {
          throw e;
        }
      }
      try {
        spanner.getDatabaseAdminClient().createDatabase("test-instance", "test-db", Arrays.asList(
               "CREATE TABLE Singers (\n"
            + "  SingerId     INT64 NOT NULL,\n"
            + "  FirstName    STRING(200),\n"
            + "  LastName     STRING(200) NOT NULL,\n"
            + "  LastModified TIMESTAMP OPTIONS (allow_commit_timestamp=true),\n"
            + ") PRIMARY KEY (SingerId)")).get();
      } catch (ExecutionException ee) {
        SpannerException e = (SpannerException) ee.getCause();
        if (e.getErrorCode() != ErrorCode.ALREADY_EXISTS) {
          throw e;
        }
      }
      final Random rnd = new Random();
      DatabaseClient client =
          spanner.getDatabaseClient(DatabaseId.of("test-project", "test-instance", "test-db"));
      client.readWriteTransaction().run(new TransactionCallable<Void>() {
        @Override
        public Void run(TransactionContext transaction) throws Exception {
          transaction.executeUpdate(Statement.newBuilder(
              "INSERT INTO Singers (SingerId, FirstName, LastName, LastModified) VALUES (@id, @first, @last, PENDING_COMMIT_TIMESTAMP())")
              .bind("id").to(rnd.nextLong()).bind("first").to("Pete").bind("last").to("Harrisson")
              .build());
          transaction.executeUpdate(Statement.newBuilder(
              "INSERT INTO Singers (SingerId, FirstName, LastName, LastModified) VALUES (@id, @first, @last, PENDING_COMMIT_TIMESTAMP())")
              .bind("id").to(rnd.nextLong()).bind("first").to("Zeke").bind("last").to("Allison")
              .build());
          return null;
        }
      });
  }
}

The same behavior is also observed if the two statements are sent as one BatchDML request.

The same operation does work on a real Spanner instance.