If an Rtf document is imported and then exported to a Docx file, every single paragraph has associated list when the document is opened in MS Word.

Workaround: Before the document export, clear the ListId property:
if (this.radRichTextBox.Document.Style.ParagraphProperties.ListId == -1)
{
    var property = this.radRichTextBox.Document.Style.ParagraphProperties.GetStyleProperty(Paragraph.ListIdProperty);
    property.ClearValue();
}