That's very odd... The API is based on SOAP XML. XML documents typically have a character set encoding specified either in the HTTP headers (e.g.: Content-Type: text/xml; charset=utf-8) and in the XML data itself (Pasted just above my signature).
Is it possible that the characters being sent are outside of the character set specified in your API request?
I'd recommend that you double check and try again. If the character set encoding is incorrect, it could explain why the characters are changing.
<?xml version="1.0" encoding="utf-8"?>
Edited: Tue April 03, 2018 at 4:39 PM by SupportRep