Migration Assistant has lost every password!

I’ve just upgaded my MBP 17” High Sierra to MBP 16” Monterey. I used Migration Assistant to move everything, with the MBP 17” in Target Disk mode.

Every current password has been lost!

Preferences -> Passwords has dug up a load of out of date passwords. The Google password was wrong, and had been changed ages ago.

Passwords for sites that I visit daily, Facebook & LinkedIn, were lost! Not in Preferences -> Passwords, nor Keychain. Many other sites, too. I used to use Keychain on the previous MBP, and it was fine.

What’s going on?

I use iCloud Keychain on an iPhone, and I can log on to sites with the iPhone that I can't log on to with the new MBP. Preferences -> Apple ID -> iCloud -> Keychain wasn't originally checked, but it is now.

Apart from that, I have to use a system password, or finger print, in addition to the site password for every site that I log on to. This will get really infuriating!

Any assistance would be very much appreciated.

Many thanks,

David