To answer your question above, no, the arm does not straighten on its own on either of my robots. The weight of the full arm when assembled seems to counteract the torsion spring. I did experience the kind of behavior to which I believe you are referring in regards to the Shoulder assembly after completing Step 2 of the v2.0 robot assembly (before the Elbow and all the parts of the Arm where attached in Step 5).
You mentioned that you found something broken when you disassembled the Shoulder. Was the 3D-printed Arm Top broken inside where the torsion spring attaches? That would probably make the torsion spring float freely inside and explain the behavior you were experiencing.
Would you please post a photograph showing what was broken? I am just interested in what was causing the problem. This would probably be of use to others on the forum in diagnosing problems that might occur with their robots.