Biplanes has a tendency to roll with rudder application due to the top wing. All of my Ultimates do this. Many planes also have a tendency to pull to the bottom or the top when flying in kinfe edge. You either have to fly it out or mix it out in the radio. I' be willing to bet your plane won't make a flat turn either exhibiting the same characteristics as in knife edge flight. I've had lots of bipes an this is common. A lot of mono wing planes do this as well, some less some more.

This rolling tendency with rudder use also enables bipes to do snap rolls faster. My DPM Ultimate will do outrageously violent and fast snap rolls as will my Eflite ultimate.