Question OnGUI only works when Game tab is visible (?)

I'm working on a cinematic cutscene in the Timeline. There's a scene where I wanted the light to flicker, and have the lamp object's material's Emission value reflect the light Intensity via script (so I don't have to manually key-frame it).

To that end, I've written a short script.
Please keep in mind that I'm mostly just an artist working on the game, so the following might contain some horrible mistakes:

Spoiler: The script

Code (CSharp):

1. using UnityEngine;
2.  
3. [ExecuteAlways] //so that it works in the Editor, which is what I want
4. public class CeilingFanLightController : MonoBehaviour
5. {
6.     //I'm gonna take the light's intensity value and multiply my Emission color by it. And a few steps in-between.
7.  
8.     public GameObject myLamp;    //putting the lamp object here so I can access its material's Emission value.
9.     public GameObject lightObject;    //putting the light object here so I can access its Light component.
10.     public Color myColor;    //base color that's gonna be multiplied. I have it as ALMOST black, so that the emission almost doesn't show when off.
11.     //if I make it pure black, then it won't brighten when multiplied.
12.  
13.     public float Multiplier = 22;   //A large enough value for the light's intensity value to be multiplied by.
14.     [SerializeField]   //so I can visually see the following in the Editor:
15.     private float MultipliedIntensity;   //equals to light intensity * Multiplier.
16.     [SerializeField]
17.     private Color finalColor;   //the color that I want to constantly* be assigned as the emissioncolor.
18.        
19.     private void OnGUI()   //*this is the CONSTANTLY part.
20.     {
21.         MultipliedIntensity = lightObject.GetComponent<Light>().intensity*Multiplier;    //so the small intensity changes become large enough to affect the color parameter.  
22.         finalColor = myColor * MultipliedIntensity;   //getting that almost-black-but-not-quite initial Color and cranking it up a bunch.
23.         myLamp.GetComponent<Renderer>().material.SetColor("_EmissionColor", finalColor);
24.     }
25. }
26.  

Even still, this works exactly as I intended! Except... for some reason, only when the Game tab is visible in the Editor.

Strangely (for me), If I change the function to Update(), then it works even if the Game is not visible.
Could anyone explain to a noob what's going on? Also, alternative more elegant solutions are welcome. Though, since I'm interested in scripting, I'd be glad to understand why my code has this unintended side-effect.

Spoiler: So, why not just use Update()?

Because Update seems to lag a frame behind (the emission changes a frame later than the intensity does), which is kind of unacceptable in this case.

Spoiler: Why can't you just use the Game mode for this?

It's very inconvenient to have to launch the game and wait for it to compile, then get to a certain point, not being able to rewind, etc.

Spoiler: Why did you make mistakes [A], [B] and [C]?

Because I'm almost a total noob when it comes to scripting. I work as a 3D generalist, but I'd like to learn more about coding and stuff.

Edit: Now that I notice it, even when using OnGUI, the emission still lags a frame behind in the Game window. It does not lag in the Editor, though.