Step to reproduce: radTreeView1.Nodes["Random Node"].Enabled = false;

WORKAROUND:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        radTreeView1.CreateNodeElement += radTreeView1_CreateNodeElement; 
        
    }

    void radTreeView1_CreateNodeElement(object sender, Telerik.WinControls.UI.CreateTreeNodeElementEventArgs e)
    {
        e.NodeElement = new MyTreeNodeElement();
    }
}

public class MyTreeNodeElement : TreeNodeElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(TreeNodeElement);
        }
    }

    protected override void InitializeFields()
    {
        base.InitializeFields();
        this.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;
    }
}