This one looks real: the MailEntityEventConverter is not expected an optional as source of the event. And I'm not sure it's actually correct that this event is triggered with an Optional: the doc says that those events are triggered with an EntityEvent. So looks like there's a bug in the AbstractAsynchronousEventStore which triggers those, here: https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-eventstream/xwiki-platform-eventstream-api/src/main/java/org/xwiki/eventstream/internal/AbstractAsynchronousEventStore.java#L468

Yes, AbstractAsynchronousEventStore#syncDeleteEventStatus, AbstractAsynchronousEventStore#syncDeleteMailEntityEvent and AbstractAsynchronousEventStore#syncDeleteEvent looks wrong. There is no reason for them to have a format different from the other AbstractAsynchronousEventStore#sync* methods. Looks like unfinished refactoring.