Force a sync of iCloud Keychain from iOS to macOS?
Solution 1:
This is how you can force an iCloud keychain sync as per Apple Support document, Get help with iCloud Keychain
If you want to sync iCloud Keychain with other devices
Make sure that your new device has the latest version of iOS and the latest version of macOS. Then, make sure that your device is on a Wi-Fi or cellular network. If the connection appears strong, try these steps:
Turn off iCloud Keychain on all devices.
Turn on iCloud Keychain on the device that has the most up-to-date keychain items.
Turn on iCloud Keychain on your other devices. They will be updated with your new iCloud Keychain.
Solution 2:
I had very little luck with the various Apple approved methods of solving this issue. Nothing would appear on my Macbook’s iCloud Keychain even after very elaborate tricks to turn iCloud Keychain off and on.
What did work was that in frustration I airdropped a single web site password from my iPhone to my Macbook. Subsequently every password appeared in iCloud Keychain on my Mac and subsequent passwords synced over iCloud. (Like the very password saved on this site I just made on my iPhone 5 minutes ago synced over iCloud to the MacBook soon thereafter).
I don’t know why this worked or if it will work for others but worth trying!