Full-size beavers add vertical fin area when floats are mounted. Sometime's there's a separate fin under the rear of the fuselage. Another mod (Certificated by Kenmore Air) is to add small fins to the end of the fixed part of the horizontal stab. Apparently the side area of the floats in front of the CG is enough to make more stabilizer area behind the CG necessary.

Also: Flat-bottom airfoils tend to exhibit much more adverse yaw than symmetrical airfoils.

With inadequate stabilizer area, and adverse yaw, ailerons will make the plane yaw the wrong way and the dihedral will then cause it to roll the WRONG way. Apparently yours is juuust balanced and the adverse yaw/roll coupling is exactly the same as the aileron roll force.

I was there for the test flight of a 100" beaver, and it needed substantial rudder mix with the ailerons to fly properly.

Program/build in plenty of aileron differential, and mix in at least 25% rudder with the aileron. Add a little more fin area. Earlier remark about being sure the ailerons have positive motion under load is a good one. Be sure your servos are mounted solidly, and the aileron horns are mounted into a reinforced place on the aileron.

I got around the problem by fudging a Senior Telemaster to make it look (mostly) like a Beaver. It still needs rudder mix with ailerons. 96" Span, 10 pounds with floats. EEEEASY to fly.