How to reproduce: 
var descriptionItem = new DescriptionTextListDataItem()
{
    Text = "Chicken wings",
    DescriptionFont = new Font("Arial", 20, FontStyle.Bold),
    DescriptionText = "some description",

};

this.radListControl1.Items.Add(descriptionItem);

Workaround: handle the VisualItemFormatting event and set the font to the DescriptionContent element
https://docs.telerik.com/devtools/winforms/dropdown-listcontrol-and-checkeddropdownlist/listcontrol/customizing-appearance/formatting-items