Forum Discussion
How did 1Password automatically know my account on a new Mac?
Swerve The issue you're concerned about is related to how Apple activates iCloud on new devices, which inevitably syncs data to iCloud before a user can manually deactivate the service via the toggles in the 'Apps Using iCloud' settings.
That said, according to Apple, "Keychain items are transferred from device to device, traveling through Apple servers, but are encrypted end-to-end so that Apple and other devices can’t read their contents."
You may wish to report feedback related to this concern to Apple on either of these pages.
https://www.apple.com/feedback/icloud.html
https://www.apple.com/feedback/macos/
The Keychain entry for 1Password contains the account key, account name, email address, the last used date, your 1Password url (custom subdomain if business, teams), and a UUID. Your 1Password password is never stored in Keychain by 1Password.