This functionality is supported by MS Word: 

A possible workaround could be to enclose the RadSpreadsheet as a UI element. More information could be found in the InlineUIContainer help article.

The DeepCopy method does not copy the font of the predefined table styles

Workaround:

var fragment = new DocumentFragment(radRichTextBox.Document);
radRichTextBox.Document = new RadDocument();
DefaultStyleSheet.Instance.ApplyStylesheetToDocument(radRichTextBox.Document);
radRichTextBox.InsertFragment(fragment);

The paste options popup stays visible when the window is deactivated

Workaround: 

After deleting a custom annotation using the DeleteAnnotationRange method when there is a text immediately in front of the annotation start:

and then trying to delete the last character in this former annotation range an exception is thrown (Exception: System.InvalidOperationException: 'inline does not belong to this document.')

When "rFonts" is set in docDefaults:

<w:styles>
	<w:docDefaults>
		<w:rPrDefault>
			<w:rPr>
				<w:rFonts w:ascii="Verdana" w:eastAsia="Times New Roman" w:hAnsi="Verdana" w:cs="Verdana"/>
				...
			</w:rPr>
		</w:rPrDefault>

and partially in the local run properties:

<w:r>
	<w:rPr>
		<w:rFonts w:cs="Times New Roman"/>
		...
	</w:rPr>
	<w:t>Text</w:t>
</w:r>

A wrong font is applied.

Expected: Verdana
Actual: Times New Roman