To reproduce: 
-open RadRichTextBox Telerik Editor from demo applications;
-clear the default content;
-change the font;
-enter some text lines, but with available empty lines between them; check if the empty lines Font is the desired one;
-select the whole content and Copy;
-clear the content and Paste; as a result the text lines has the desired Font, but the empty lines have the default "Calibri" Font;

Alert: This is an issue provoked by RadDocument. RadDocument is a very fundamental element in the RadRichTextBox that is why the visibility of the issue is bigger than just a copy paste problem.

Workaround:
Use KeyDown an KeyUp event (handling Copy/Paste shortcuts). On KeyDown event the user can export RadDocument content and on KeyUp event to replace the clipboard content with the exported one.

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