I have the same problem in a couple of planes I have designed. One is a small scale Spacewalker which acts totally different from my other 1/4 scale version. When rudder is applied there is a definite pitch down effect. Stall turns have to be completed with a lot of up elevator otherwise the turn gets out of plane due to the pitch down. Knife edge is just as troublesome with pich away from the canopy with rudder application.

The spacewalker has a semi symmetrical wing aerofoil and flat plate (sheet) tail surfaces.

Any thoughts gratefully received.