Complete soap request is replaced by propery expansion

I have very strange behaviour in some of my tests.

The Soap XML request is like this:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:itf="http://itf.classic.synchronisation.it.koopmanint.com">
   <soapenv:Header/>
   <soapenv:Body>
      <itf:errorRetry>
         <fileName/>
         <recordNumber>${Retrieve first SyncError (IKR)#ResponseAsXml#//Results[1]/ResultSet[1]/Row[1]/SYNC_ERROR_ID[1]}</recordNumber>
      </itf:errorRetry>
   </soapenv:Body>
</soapenv:Envelope>

When this teststep is run as part of a testcase, the property expansion gives the following raw request:

POST http://nidxprd:9080/ClassicSyncServices/services/ClassicErrorRetryItf HTTP/1.1
Accept-Encoding: gzip,deflate
Content-Type: text/xml;charset=UTF-8
SOAPAction: ""
User-Agent: Jakarta Commons-HttpClient/3.1
Host: nidxprd:9080
Content-Length: 5

21570

As you see, the complete XML reuquest is replaced by the expanded property.
If I then retry the single step only, the property expansion goes OK, and the intended soap request can be seen in the Raw request:

POST http://nidxprd:9080/ClassicSyncServices/services/ClassicErrorRetryItf HTTP/1.1
Accept-Encoding: gzip,deflate
Content-Type: text/xml;charset=UTF-8
SOAPAction: ""
User-Agent: Jakarta Commons-HttpClient/3.1
Host: nidxprd:9080
Content-Length: 331

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:itf="http://itf.classic.synchronisation.it.koopmanint.com">
   <soapenv:Header/>
   <soapenv:Body>
      <itf:errorRetry>
         <fileName/>
         <recordNumber>21570</recordNumber>
      </itf:errorRetry>
   </soapenv:Body>
</soapenv:Envelope>

Although I can reproduce this if I run again, I have similar requests that do not have this behaviour.