pollitoyeye

It is not fixed.

It goes dark or light depending of where it's placed.

What I am riding in the picutres is made of moving armorstands so it is a good way to test it, you can notice the problem in the wheels.

It should always look like the first picture.

Confirmed for 1.15.1

As you asked I did it. It took 10m.
Haven't tested it yet but if this is not the solution it's something really near from this.
Put this on EntityArmorStand.class. (Names mapped from Forge 1.12.2)

@Override
    @SideOnly(Side.CLIENT)
    public int getBrightnessForRender(){
    	if(hasMarker()){
    		return 15728880;
    	}
    	return super.getBrightnessForRender();
    }

If you actually want the armorstand to render darker depending of the blocks light it would be as easy as changing the inside of the if and taking in consideration external block lights.
As it's overriding a super class method and only gets called for marker armorstands this breaks nothing. Even a new tag could be added just for this as mentioned. NoShadow for example.

I have to disagree with you. Making entities with a tag rendering no shadows is not something difficult.
It is as simple as modifying the current tag to work like that or adding a new tag "NoShadow" that removes entity shadows.
You can't sell something that probably implies an if -> return as a work of titanic magnitudes.

I can accept the fact that they won't just fix it because they don't even remember this issue or they are with other things they "consider more important".
But please, don't threat the most important part of the community (Map Makers) as if they were stupid.

We entered in 2018 and an important issue opened in 2016 that was caused by the programmers isn't even fixed yet.
I see bug fixes every update that are totally less relevant than this one.
31 persons voted for this issue and the only response we got was "use another blocks with resourcepacks".
Armorstands were also introduced to evade resourcepacks and allow good vanilla creations.
What's the point of them if you suggest us to use resourcepacks?
As a programmer I'm totally sure fixing this won't take more than a day as other devs even pointed how to fix this.
Seeing how this issue gets marked as still happening every snapshot is just sad.