To reproduce:
1. Add two RadButton controls and use the following code:
public Form1()
{
    InitializeComponent();

    this.radButton1.Font = new Font("Verdana",8.25f, GraphicsUnit.Point);
    this.radButton1.Enabled = false;
    this.radButton1.ButtonElement.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;

    this.radButton2.Font = new Font("Segoe UI",8.25f, GraphicsUnit.Point);
    this.radButton2.ButtonElement.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;
    this.radButton2.Enabled = false;
}
2. If you zoom enough to investigate the exact text rendering, you will notice that the first button does not take into consideration the TextRenderingHint.AntiAlias.

Resolution: Add the DisabledTextRenderingHint property which allow users to specify a text rendering hint that will be used in disabled states