Why won't Safari save passwords to iCloud

I've been using iCloud passwords for a long time with no issues. This week, suddenly Safari won't remember any new passwords. It still auto-fills previously saved passwords though. In Safari preferences, the box is checked for auto-fill passwords. When I register for a new website, Safari generates a random password as usual, and it says it will remember it in the iCloud keychains, but it doesn't.

I've tried Keychain First Aid, in the Keychain Access app. No problems detected. In Keychain Access, it shows both iCloud and login keychains unlocked. I don't know what the problem could be.