Arms Over-Rotate When Using IK While Bending Spine

I tried using Unity's built-in inverse kinematics system to aim a gun, and it worked perfectly until I decided to make the spine bones look in the opposite direction that the camera is facing in order to make the character lean forward and back so that the arm holding the gun doesn't clip through the character's torso. Once I did this, I noticed that the character's hands would rotate beyond their IK targets. I then tried using DitzelGames' IK script, instead, and didn't have that problem. So, is this a bug, or was I just missing something while using the built-in IK system? I would like to use the built-in one. But, for anyone who may be interested, here is a link to DitzelGames' one: https://assetstore.unity.com/packages/tools/animation/fast-ik-139972