A airplane with a symmetrical fuse in the vertical plane and a mid mounted wing has no roll coupling. Any divergence from that configuration will cause roll in knife edge. It comes from the Clbeta (rolling moment due to side slip) aero term. The symmetrical airplane has a zero Clbeta.

The Ultimate biplane configuration, as good as it is otherwise, has a high wing that messed up the symmetry and it ends up having a value for Clbeta. Do a knife edge and it will roll. I remember seeing a design that had anhedral in the upper wing to help stop the effect. It looked pretty bad.