To reproduce:
            radTimePicker1.Enabled = false;
            radTimePicker1.Enabled = true;

WORKAROUND:
 void radTimePicker1_EnabledChanged(object sender, EventArgs e)
{
    if (radTimePicker1.Enabled)
    {
        radTimePicker1.TimePickerElement.ForeColor = Color.Black;
    }
}