Not really a bug, just more of a UI design issue. When you add a new Custom or Scripted Function from the Triggers screen, it gets created in the same place as the other functions on the Settings > Functions tab. If you switch to that tab, you should see it there and you can edit it. I've put this on our list to review for the next version as well, as I think we should have a way to edit the functions right from the Triggers tab
