I have just tried and you're right it doesn't work (same SA build than you).
I suspect a bug or maybe it's a "normal" behavior with Preview Mode...
An answer from the Sencha Team should be very appreciated.
P.S: You can set up a virtual folder with IIS or Apache (or any other..) pointing to your SA project folder.
Like this you have just to save your project and access the local url via your browser. By this way the _dc param is always removed (as long as you have uncheck the config in SA).