SystemGroup (SystemRoot) only calls the first function added to it.

[Isolyth]

public override void _Ready()
	{
		EntityManager.Initialize();
		root = new(EntityManager.Store){
			new SpriteRenderingSystem(),
			new SpriteRIDAdder()
		};
		var store = EntityManager.Store;
		store.CreateEntity();
		store.CreateEntity();
		var sprite = store.CreateEntity();
		sprite.AddComponent(new RCPosition{Position = new(0,256)});
		sprite.AddComponent(new RCRotation{Angle = 14});
		sprite.AddComponent(new RCSprite{Texture = ResourceLoader.Load<Texture2D>("res://icon.svg"), Dirty = true});
		sprite.AddComponent(new RCSpriteRID());
		
	}

	// Called every frame. 'delta' is the elapsed time since the previous frame.
	public override void _Process(double delta)
	{
		acc += delta;

		if (acc >= 0.5){
			root.Update(default);
			acc = 0;
		}
	}

Only the first system gets called, I have tested with swapping the location of both systems, and in both cases only the first system will be the called, the second will not.

[friflo]

Can you add a perf log after instantiation and Update() to see the system hierarchy?
Maybe the second system is disabled or an exception is thrown by the first system.

    GD.Print(root.GetPerfLog());

The setup of systems is a very basic operation.
In issue like this should have already been reported and seen in unit tests.