To reproduce:
var stackPanel = new StackLayoutElement { Orientation = Orientation.Vertical };
stackPanel.Children.Add(new TextPrimitive { Text = "Demo" });
 stackPanel.Children.Add(new TextPrimitive { Text = "Demo" });
Panel.PanelElement.Children.Add(stackPanel);
Panel.AutoSize = true; // <= false works fine

Workaund:
- Put the panel in a control:
public class MyControl : RadControl
{
    protected override void CreateChildItems(RadElement parent)
    {
        base.CreateChildItems(parent);
        
        var stackPanel = new StackLayoutElement { Orientation = Orientation.Vertical };
        stackPanel.Children.Add(new TextPrimitive { Text = "Demo" });
        stackPanel.Children.Add(new TextPrimitive { Text = "Demo" });

        parent.Children.Add(stackPanel);
    }

    protected override RootRadElement CreateRootElement()
    {
        RootRadElement root = base.CreateRootElement();
        root.BackColor = Color.Transparent;

        return root;
    }
}