In the Team Calendars admin page, there's a tab that allows you to rebuild space calendars, which allows you to re-run the migration to calendars in spaces. The tab doesn't currently provide any information about what this'll do or why you'd do it. The current title is Rebuild Space Calendars, and the accompanying text is Rebuild Space Calendars.

Proposed text:

Calendars that have a related space set, or are embedded on only one page, should appear in the Calendars section of the relevant space. If the calendars don’t appear there, rebuild space calendars to run the migration process again.