Excel cannot see Windows mapped drive when running under a service

I have an application that runs as a service. This application allows the user to connect and starts a subprocess under the user's windows account. In other words, mydomain\john connects to the service via TCP and says "launch" which starts a process on windows running under mydomain\john account.

Next, this subprocess is able to map drive T: using net use command. Then the process can run a VB script living on the mapped drive T:. No trouble so far. the problem arises when this VB script tries to run Excel using "createObject".

After setting DCOM launch settings with DCOMCNFG and creating the missing "Desktop" folder in the systemprofile folder, I still cannot get this to work. Oddly, if run my vb script and feed Excel a UNC path \fileserver\share1 rather than T:\ the script works.

Does anyone know why Excel would fail to access the T:\ mapping but have no issue with the UNC path?