I've just taken a look at the documentation for TextWriterTraceListener and there's a note about 1/3 of the way down the page
If an attempt is made to write to a file that is in use or unavailable, the file name is automatically prefixed by a GUID
So, this would appear to be by design. If the file is indeed unavailable then there's nothing that can be done about it with the current implementation. What you could try doing is writing a custom implementation of TextWriterTraceListener that overrides the relevant Write/WriteLine methods so that the output goes to a file, per user, with a name that better suits your needs.
If what you want is for ALL logging from ALL users on the Terminal Server to go to a single file, then you'll almost certainly need to have some kind of "3rd party" process running that "owns" the file and synchronises writes to it, such as a Windows Service that is then called by your custom TextWriterTraceListener