To reproduce:
- Add several paragraphs with  SpacingAfter set to 0.
- Export and then import using XAML format provider.
- You will notice that the spacing is reset to the default value.

Workaround:
Paragraph para = new Paragraph();
para.SpacingAfter = 0.01f ;

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.