I think you are on the right track with the reason for this bug, try this with steph petite on the right shin bend, rotate the inner sphere Z rotate from 180 to 0 now do a symmetry from right to left, this flips the zrotate from 0 to 180  on the left leg but also this snaps the falloffs into alignment in poser. the key is that both the X and Z rotations were on or close to 180 degrees in one sphere which is bad, this causes the gimble lock issue.