The ltpatoken is stored in memory only and according to the MS doc above "Cookies that do not have an expiration date are stored in memory and are available only to the process in which they were created.".
I suggest two possibilities:
1. the method call is not passing a large enough buffer to accept all the cookie data - are you catching the reply code for the method call? what is it? eg http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1893982&SiteID=1 The ltpatoken cookie is pretty big ;-)
2. There is a bug in the MS code that somehow fails if one of the cookies is an in memory one.