Planes like Extra and Cap by design will go to belly at knife edge. When the thrust line is high above and center of drag (close to the wing leading edge) is down below, it will naturally pull it downward. When in level flight, the weight will help compensating for the downward moment. But in knife edge flight, the weight vector is no longer working against this downward moment, but rather causing pitch moment (if left side fuse is up, it will roll to the right) because the center of mass is closer to the canopy than the center of lift of fuse side. I had both of the low wing Extra and Cap, and my findings support this theory.

Conversely, a high wing plane will pitch to the canopy and roll to level during knife edge.

With the Dave Patrick Ultimate, the center of drag is about co-axial with the thrust line, so they advertise "no pitch coupling in knife edge". But its up-down center of mass is closer to the canopy than the fuse center of lift, so it will have pitch coupling at knife edge, rolling toward inverted. It sorts of makes sense. Same with the true mid wing planes (with mid thrust-line).

For your Extra, to reduce pitching to the belly during knife edge, reduce down thrust. I am not sure about adding nose weight or tail weight. I haven't gotten the theory behind that figured out yet.