I don't if this should be called a bug. If not, maybe the topic could be moved to the suggestions forum. Or maybe it's not at all possible to make it work. Anyway, here's the thing:

One great advantage when using the Rich Text Editor is when you copy a linked word or phrase into the post, you will get in right in there at once.
This however doesn't work when using internal links since the board url part will be missing in the link. Ex:
http://index.php/?showforum=15

Maybe it would be possible to have the full URL's in posts when posting internal links this way?

This is not a bug in IPB but is more of a feature suggestion.