iCloud Keychain will not sync down to macOS

Late 2009 iMac, macOS High Sierra 10.13.6

As of roughly 1 month ago, my Mac no longer seems to correctly pull passwords and saved information from my keychain down from iCloud. This first became apparent when after some system update (sadly I did not document which one), Safari no longer showed any stored passwords. All passwords saved/updated on my iMac are indeed seen by my other iCloud connected devices (iPhone 6s, iPhone 8, last year's low end iPad); however passwords saved/updated on those devices are not seen on the iMac. Also, my iCloud keychain in Keychain Access does not appear to pull down from iCloud, but this is less clear to me.

Is anyone aware of a fix for this? This issue has prompted me to explore Keychain Access in more depth than I have before, and I'm wondering if there is something I can blow out there (or in a backend directory) that will retrigger the correct syncing process between my iMac and iCloud. Any thoughts?

(Before anyone asks, I am aware my 2009 iMac is no longer supported by Apple.)