Trouble using an animation C# job to aim the weapon (pitch only)

Unity 2018.3.0f2

I'm using PlayableGraph to control the animation of my character. I have a basic clip for the character holding a gun aiming. I want to use the pitch angle to correct the direction of the gun. In the following Job, I rotate the shoulder to aim the gun correctly, but what I see on the screen is the shoulder jumping forever between two rotations.

Code (CSharp):

1. public struct AnimJob_WeaponLookAt : IAnimationJob
2.     {
3.         // Pitch angle.
4.         public float Pitch;
5.  
6.         // The shoulder whose hand holds the weapon.
7.         public TransformStreamHandle HoldingShoulder;
8.  
9.         // Root node of the weapon.
10.         public TransformSceneHandle WeaponTransform;
11.  
12.         // Root node of the character.
13.         public TransformSceneHandle RootTransform;
14.  
15.         public void ProcessAnimation(AnimationStream stream)
16.         {
17.             var input = stream.GetInputStream(0);
18.             var pitchRot = Quaternion.Euler(Pitch, 0, 0);
19.             var weaponSrcRot = WeaponTransform.GetRotation(input);
20.             var weaponFromDir = weaponSrcRot * Vector3.forward;
21.             var weaponToDir = pitchRot * RootTransform.GetRotation(input) * Vector3.forward;
22.             var shoulderOriginalRot = HoldingShoulder.GetRotation(input);
23.             var shoulderRotDelta = Quaternion.FromToRotation(weaponFromDir, weaponToDir);
24.             HoldingShoulder.SetRotation(stream, shoulderRotDelta * shoulderOriginalRot);
25.         }
26.  
27.         public void ProcessRootMotion(AnimationStream stream)
28.         {
29.             // Empty.
30.         }
31.     }

Anyone could give me some advice, please? Thanks.

Notes:
- The gun has no animation. It's just a prefab attached to a point at the character's hand.
- The character is humanoid.