When a customer preps their cloud site by installing an app they want to migrate, the add-on user is added to the cloud site. After setting up their cloud site, when they migrate space and page content via the Confluence Cloud Migrations Assistant (CCMA) or via manual import of the Space, the add-on permissions are not updated for the imported space. This means the add-on will work in new spaces but not in migrated spaces.
As a workaround, the customer would need to re-install the app on the cloud site post migration to have the add-on user added to space permissions for all spaces. Or update the space add-on permissions.