I'm having the same question.
Just for clarification: There's 2 types of events:
Normal event invitations (usually via email) - you can Accept, Tentative or Decline these and once you made a selection, they show up in the "Replied" tab.
Siri event suggestions (automatically generated from e.g. email contents, messages, etc.) - you can Accept (add to Calendar) or Ignore these. Once you made a selection, they are gone from Calendar's Inbox or Replied list.
As a workaround, you can find the email or message where this suggestion was coming from and long-press the date/time in that message. This should allow you to create an event based on that time.