This is not a Twonky problem. The same problem occurs when using TVersity or WMP as the UPnP server. It's a known bug in the firmware of the WAS7500. The WACS6050 works correctly so Philips have the code, but as of FW V1.26 this is still unresolved in the WAS7500.
If you have issues or questions with the WAS7500 you'd be well suited to heading over to http://www.streamiumcafe.com
which is THE best place for all information related to the Philips Streamium products.