I've asked Siri to add an event to my calendar. When I ask Siri "show me my calendar for Monday", the event appears:
but the event doesn't actually appear in the calendar application:
I use my Google calendar as my default calendar, but I don't think that should make a difference. The event doesn't appear on any other device.
I have all the calendars enabled. It looks like it has added the event to the 'iCloud - Work' calendar:
UPDATE: I received an alert reminding me of the event that morning, and since then the event appears in the calendar: