>>2941317 (OP)
bracing is useful against torsional moments
what it cannot do is make up for bowing or bending
i suggest your legs are too thin
adding more bracing might help, not because its reducing torsion but because you might accidentally add a horizontal member between legs at the half way point which will reduce bending.
the other point is that your fixings are all single nail? a triangle is strong because for any given 3 fixed length sides, the angles can't change. unlike a square can turn into a rhombus. but if your legs are bending then the effective length of the triangle sides can/does change and therefore the angle can change. if you added more nails or screws at the joins it would impede the change in angle which would strengthen the bending member.