The UI indicates that a strikethrough is applied, but it is not visible in the document. 

Workaround: 

Reorder the UI layers of RadRichTextBox so that the one responsible for drawing the strikethrough decoration is added in a higher position: 

protected override void BuildUILayersOverride(IUILayerContainer uiLayerContainer)
{
    uiLayerContainer.UILayers.Remove(DefaultUILayers.StrikethroughDecoration);
    uiLayerContainer.UILayers.AddAfter(DefaultUILayers.UnderlineDecoration, new StrikethroughTextDecorationUILayer());
}