I believe I have found a bug in the IDE. Open a new project, then drag any control onto the main window from the library. Next double click the control and add any event handler. Then click on the control in the inspector and add it to a new control set. Next hit command-Z (or whatever your undo keystroke is) to undo adding it to the control set. Now try and re-add the event handler from before. It won’t be there. This happened to me yesterday with one of my projects, and I was forced to quit without saving to get the code back from the events, causing me to loose several unsaved changes. Needless to say, this was a major inconvenience.
I would submit a feedback case, but I am not exactly sure how to do so. If someone can confirm this then send in a feedback case, that would be excellent.