Tab Control bug
When you cut controls in a form & paste them into a tab control they lose all of their links to event procedures.
Gerrit Viehmann commented
This will also happen when controls are moved between Header/Footer or Body.
E. g. if a table-layout is applied to controls with events, the event-properties containing "[Event Procedure]" may be cleared.
I've also seen this happening without a corresponding action. In those cases all events in a form became unlinked.
Remedy: If you select the whole affected Module and press [Tab] and [Shift]+[Tab] (indent and unindent) the missing "[Event Procedure]"-properties will magically reappear.