To reproduce:
- Open a document that contains symbols with the 2015 Q3 SP1 version.
- Export the document using the docx format.
- Open it again.

Workaround:
private void radRichTextEditor1_DocumentChanged(object sender, EventArgs e)
{
    this.SelectAllMatches("");
    this.radRichTextEditor1.ChangeFontFamily(new Telerik.WinControls.RichTextEditor.UI.FontFamily("Symbol"));
    this.SelectAllMatches("");
    this.radRichTextEditor1.ChangeFontFamily(new Telerik.WinControls.RichTextEditor.UI.FontFamily("Symbol"));
}
 
private void SelectAllMatches(string toSearch)
{
    this.radRichTextEditor1.Document.Selection.Clear();
    DocumentTextSearch search = new DocumentTextSearch(this.radRichTextEditor1.Document);
    foreach (var textRange in search.FindAll(toSearch))
    {
        this.radRichTextEditor1.Document.Selection.AddSelectionStart(textRange.StartPosition);
        this.radRichTextEditor1.Document.Selection.AddSelectionEnd(textRange.EndPosition);
    }
}