When a document is imported from HTML, we set locally to the span elements Times New Roman as a font family nevertheless it is not declared anywhere. The same applies for the font size as well - it is set to 12.

Workaround: Clear the FontFamily property after import

foreach (Span span in this.radRichTextBox.Document.EnumerateChildrenOfType<Span>())
{
    span.GetStyleProperty(Span.FontFamilyProperty).ClearValue();
    span.GetStyleProperty(Span.FontSizeProperty).ClearValue();
}