Many planes will roll or pitch with rudder in knife edge. (pitch and roll coupling)

The amount of rudder you apply to hold the knife edge affects how much pitch and roll effect you get.

And how much airspeed and how you have the fuse angled as you roll to knife edge will affect how much rudder you wind up trying to apply.

How vertical your knife edge affects things as well. I used to "cheat" my knife edges a bit on my Cap by rulling a bit past vertical, it looked good, and reduced some of the coupling.

So, first thing to do is to get as consistant as you can at entering knife edge. If you can't roll in consistantly, you'll drive yourself nuts trying to hold it.

With work, you can just hold in the approprate amount of rudder and aileron to keep the plane going straight. My Cap would roll with rudder and pitch towards the gear. I had to hold rudder, opposite aileron, and up elevator in knife edge. Took some doing, but keep working at it, you'll get better at it.

You can also mess with mixing on your radio. But until you can roll in to knife edge consistantly, it will be hard to figure out what the plane needs. Then go up high, roll to knife edge one way, and hit rudder only, and go hands-off on aileron and elevator. See what it does. Now, repeat, rolling the other way. From that, you can determine what mixes to set up to straighten the plane out a bit. Personally, I find the rudder->aileron mix to get rid of the roll coupling more useful, the pitch coupling I can handle on my own easily.

As a silly thing, you didn't accidentally turn on some kind of mix while setting up the rates, did you? On the ground, check your plane at high and low rates, move the rudder alone and see if anything else moves. Just a thought.