Custom events cannot be overriden in child blueprints. The add event option is no longer available so as to override a custom event that was created in the parent.
This appears to be a regression. Worked in 4.16.2, No longer works in 4.17 preview 2
Result: There is no option to "Add Event" as there was in the previous version. The user only has access to "Call function"
Expected: The user would be able to override the event in the child as was available in previous versions of the engine
Head over to the existing Questions & Answers thread and let us know what's up.