In some cases when trace/debug is enabled, streams have to be read, logged and then generated again. It's not unthinkable there is a bug in this scenario somewhere. If you can file a ticket with a test case we could have a look.
I found out what is wrong: When setting the logging to trace in your modeller the soap body is cleared for some unknown reason. I am not sure if this happens all the time, but this happens atleast when calling a webservice over https.
I will submit a bug report.