I actually never saw it, but after reading up on google it seems like the webservice you are calling is upholding an RPC/encoded like structure. Allthough according to https://forum.mendix.com/questions/168/What-webservice-standards-does-Mendix-support Mendix does support RPC/encoded. If you find that it is not working accordingly i suggest you file a bug report.
In order to fix this issue decently without a bug fix i would sugest to use an xslt to transform your response xml to a more Document/literal kind of style.