There’s a bug with
leg_3jnt_01 spring solver. This exists in 3.6.0 and 3.7.9.
If you reverse the angles, you get problems. This only happens with the spring solver. (Which I realize is a solver that has plenty of problems. But I really think this particular bug can be solved.)
In this image, my intended limb shape is on the left. The way that
leg_3jnt is designed to work is on the right.
In the angle calculations, it seems there is some assumption about the direction the angles must face. And when the rig builds, the one on the right is completely mangled and doesn’t function correctly at all.
(I’ll dig further and see if I can find why it’s happening.)