How to clamp rotation, following Unity's quaternion rules

I want to clamp my camera's pitch (rotation around its x-axis), and nearly every forum post or Unity Answers page says to use this method:

  1. Grab the Euler angle for the angle to be clamped.
  2. Clamp using Math.Clamp().
  3. Assign the new angle to the rotation.

However, this method is precisely what Unity advises not to do in their Quaternion and Euler Rotation article.

After some more searching, I found another answer that points to Unity's own MouseLook.cs script in the Standard Assets. This script uses the following function (full code in the link):

 Quaternion ClampRotationAroundXAxis(Quaternion q)
 {
     q.x /= q.w;
     q.y /= q.w;
     q.z /= q.w;
     q.w = 1.0f;
  
     float angleX = 2.0f * Mathf.Rad2Deg * Mathf.Atan (q.x);
     angleX = Mathf.Clamp (angleX, MinimumX, MaximumX);
     q.x = Mathf.Tan (0.5f * Mathf.Deg2Rad * angleX);
  
     return q;
 }

I've implemented this method in my own code, with a slight alteration explained in the comments in the code.

 public void Rotate(Vector3 input)
 {
     // The camera's pitch (xRotation) should be done about its own x-axis (Space.Self),
     // but its pan (yRotation) should be down about the world's y-axis (Space.World).
     // In order to accomplish this, I need to rotate the camera twice, isolating the x- and y-rotations.
     Vector3 xRotation = new Vector3(input.x, 0, 0);
     Vector3 yRotation = new Vector3(0, input.y, 0);
     transform.Rotate(xRotation, Space.Self);
     transform.Rotate(yRotation, Space.World);
 
     // Clamp the xRotation amount - Bad way
     //Vector3 currentRotation = transform.rotation.eulerAngles;
     //currentRotation.x = Mathf.Clamp(currentRotation.x, pitchMin, pitchMax);
     //transform.rotation = Quaternion.Euler(currentRotation);
 
     // Clamp the xRotation amount - Good way
     transform.rotation = ClampRotationAroundXAxis(transform.rotation);
 }
 
 // This method is taken in full from Unity's MouseLook.cs class in the Standard Assets.
 private Quaternion ClampRotationAroundXAxis(Quaternion q)
 {
     q.x /= q.w;
     q.y /= q.w;
     q.z /= q.w;
     q.w = 1.0f;
 
     float angleX = 2.0f * Mathf.Rad2Deg * Mathf.Atan(q.x);
     angleX = Mathf.Clamp(angleX, pitchMin, pitchMax);
     q.x = Mathf.Tan(0.5f * Mathf.Deg2Rad * angleX);
 
     return q;
 }

However, an odd behavior occurs when I try to rotate the camera beyond the clamp: the camera begins to roll about its z-axis. (GIF included below.) What's going on here, and how do I fix it? Is it caused by my two-rotation solution?