Unique body content when load testing with multiple threads

Using open source SOAP UI 5.5.0 on Windows 10 Pro

I have a REST API which allows me to send a file in base64 encoded form and provide a filename.  My POST request looks similar to:

<?xml version="1.0" encoding="UTF-8"?>
  <FileSendRequest>
  <FileBytes>

    PE1lc3NhZ2... (BASE64 encoded string - truncated for brevity)

  </FileBytes>
  <FileName>filename.xml</FileName>
</FileSendRequest>

When I run this manually it works perfectly.  In order to load test I created a test suite and added in a groovy script test step that runs before the POST request with the following:

ThreadIndex = context.ThreadIndex
RunCount = context.RunCount

FileName = "FileName_" + ThreadIndex + "_" + RunCount + ".xml"

testRunner.testCase.testSuite.project.setPropertyValue( "FileName", "${FileName}" )

And in the request changed:

<FileName>filename.xml</FileName>

To be:

<FileName>${#Project#FileName}</FileName>

When I run the load test with one thread and 100 requests it works correctly generating 100 files with filenames:

FileName_0_0.xml

FileName_0_1.xml

FileName_0_2.xml and so on...

If I increase the number of threads to 2 then I see filenames:

FileName_0_0.xml

FileName_1_0.xml

FileName_0_1.xml

FileName_1_1.xml so the filenaming appears to be working as I expected.

i.e. I want a unique filename for each instance of the running thread

However my test or the API seems to lose requests or files, once I increase the thread count in the load test to 2 or more.

With 2 threads, 200ms delay and 100 requests in total I see 1-2 files missing.  This loss increases as I increase the total number of runs or threads.

With 1000 requests I see a loss of ~9 files

I am doing a basic assertion of checking for a 200 response code and that never fails.

The issue could be my testing infrastructure or the API itself of course, but I'm not sure how to debug any further (I'm a little new to SOAP UI). Can anyone advise if my test is theoretically sound?  The API developer is saying that there is a clash of filenames but I don't understand how that could be when using the script as above.  Is there a better way to achieve the same thing?

Thanks