Quaternion.AngleAxis with transform.up unexpected behavior...

Here's sample code that shows my problem:

 using UnityEngine;
 using System.Collections;
 
 public class RotationAngleAxisCtrl : MonoBehaviour
 {
     public float _angleCurrent = 0.0f; // in degrees
     public float _angleSpeed = 0.0f; // in degreees/sec
 
     void Update()
     {
         Debug.DrawRay(this.transform.position, this.transform.up, Color.green);
 
         _angleCurrent += _angleSpeed * Time.deltaTime;
         _angleCurrent = _angleCurrent % 360.0f;
 
         Quaternion rot = Quaternion.AngleAxis(_angleCurrent, this.transform.up);
         Debug.DrawRay(this.transform.position, rot*Vector3.forward, Color.blue);
     }
 }

If you attach this script to a gameobject with no rotations, (0, 0, 0) in Unity Inspector, everything is working as I expect. When you press play on Unity, you will see a green ray that shows gameobject up direction and a blue ray that shows gameobject forward direction with 0 angle rotation on gameobject up direction (pic 1). If you change gameobject Y local rotation using local rotation gizmos on Unity editor window, the blue ray, correctly, does NOT rotate (pic 2). If you change the _anglespeed variable, the blue ray will constantly spin on gameobject local X/Z plane. This spin is, correctly, NOT influenced by changing value of gameobject Y local rotation on Unity editor window. Everything is perfect.

My problems arise when I start rotating the gameobject arbitrarily. For example, if I rotate the object like this (45, 0, 0), the blue ray is no more perpendicular with transform.up vector (the green ray). Why (pic 3)? If I change the _anglespeed variable, the blue ray will NOT constantly spin on gameobject local X/Z plane but in a strange circular fashion. Why?

How can I display the blue vector considering only rotations on local X and Z axis and ignoring rotations on local Y axis (since _angleCurrent will be used)?

P.S. Replacing rot*Vector3.forward with rot*transform.forward will not work since since the blue ray will include the local Y axis rotation that I want to exclude from it.