This puzzled me as well, for awhile. Try this: after you attempt to add the system, and nothing appears to happen (i.e. the list doesn't update) ... type in a restriction such as 20 LY, and click 'show'. I believe the system *is* added, you just have to refresh the display.
That seemed to be it. The system IS added, but it cannot be used in the nav comp by any ship before the refresh (which isn't always needed).
Still a bug, in my book, but not the one I thought.