Still odd - unless redirected by CALIBRE_TEMP_DIR, I would expect the temporary folder to be created at %USERPROFILE%\AppData\Local\Temp
I can't recall where where calibre puts its default library, maybe %USERPROFILE%\Documents, which is a long way from %USERPROFILE%\AppData\Local\Temp. But wherever it is, afaik, a server started via cmd doesn't remove its temp folder when the cmd process is terminated.
Don't get too emotionally attached to the server as it is now, the new one could be quite different
BR