Ahh sorry, I see now you tried with showNormal()...
I use showNormal on a dialog which I took off its title bar and handled all that my self.
For me showNormal behaves just like it would with a normal window, meaning, if I maximize the dialog, then minimize it and then resotre it, it will come back to be maximized.
Check if you are not calling showNormal twise after minimized state, this could have the effect your are experiancing, meaning, once it restores(to maximized), and the scond call restores it back to the not maximize size...