When I rotate my player in open 3 space, the transform rotation component keeps snapping back to origin. However, when I disable my rigidbody.rotate command, the transform rotation works just fine.

Here is my code:

var speed is defined by the inspector.

void FixedUpdate() {

     float moveHorizontal = Input.GetAxis( "Horizontal" );
     float moveFoward = Input.GetAxis( "Vertical" );
     float moveVertical = Input.GetAxis( "Jump" );

     float rotatePlayerYaw = Input.GetAxis( "Mouse Y" );
     float rotatePlayerPitch = Input.GetAxis( "Mouse X" );

     Vector3 rotation = new Vector3( -rotatePlayerYaw, rotatePlayerPitch, 0.0f ); 

     Vector3 movement = new Vector3( moveHorizontal, -moveVertical, moveFoward);
     rigidbody.velocity = movement * speed;

     // for debugging purposes
     //if( Input.GetKeyDown( "j" ) )
     //{
     //    transform.Rotate( Vector3.up * speed * Time.deltaTime );
     //    Debug.Log( "Key 'j' has been pressed" );
     //    Debug.Log( transform.eulerAngles );
     //}

 
     boundary.xMin = GameObject.Find( "Wall X Negative" ).transform.position.x + 15f;
     boundary.xMax = GameObject.Find( "Wall X Positive" ).transform.position.x - 15f;
     boundary.zMin = GameObject.Find( "Wall Z Negative" ).transform.position.z + 15f;
     boundary.zMax = GameObject.Find( "Wall Z Positive" ).transform.position.z - 15f;

     rigidbody.position = new Vector3
     (
         Mathf.Clamp( rigidbody.position.x, boundary.xMin, boundary.xMax ),
         Mathf.Clamp( rigidbody.position.y, boundary.yMin, boundary.yMax ),
         Mathf.Clamp( rigidbody.position.z, boundary.zMin, boundary.zMax )
     );

     rigidbody.rotation = Quaternion.Euler( rigidbody.velocity.z * tilt, 0.0f, rigidbody.velocity.x * -tilt * 1.3f );

     
     transform.Rotate( rotation, Time.fixedDeltaTime * speed * 3.0f, Space.Self );

 }