parfeval diaries sometimes missing

I have a script that runs parfeval on a bunch tasks. The code is below. The code has two outputs:

1. Results in an Excel file (one row per task)
2. A log file which gets appended with the diary from each worker.

Once all tasks have been completed, I have a complete set of results in the Excel file, but I'm missing some logs. The attached file is the list of tests reported in the log. There should be 59 rows, but there are only 50. Any idea why some diaries aren't being written to the file? My understanding is that it shouldn't be a file access issue because the second for loop is not executed in parallel. So the log file is opened, appended and closed as each worker finishes a task. Could it be a flushing issue?

Edit: I should add that if I run the job in single tasking mode everything is fine. All logs are present.

Code:

    for i = casesToRun
        f(i) = parfeval(p, @doStuff, 2, i, j, test_cases, HitsToAutoTRFile, FailureType, Standard, ModelAssembley);
    end 

    for i = casesToRun
        [completedIdx, status, xlRow] = fetchNext(f);
        writeWorkerLog(f(i).Diary, logFileName);
        xlswrite(TestCasesFile,status,'Sheet1',xlRow);
    end

    function writeWorkerLog(logData, fName)
        % create the file if it doesn't exist, then close it
       fid=fopen(fName, 'a+');
       fprintf(fid, logData);
       fclose(fid);