Safari Bookmarks Sync Issues

I found that my M4 Mac Mini, M2 MacBook Air, iPad Air (2024), and iPhone 15 Pro Max, are not syncing Safar bookmarks. All are running the latest versions of their respective operating systems.

This problem got worse when I followed the published advice so that most of my bookmarks disappeared completely when syncing with the cloud.

I restored my bookmarks file from Time Machine and also kept a copy readily available in Documents.

Now, whenever I add a bookmark, they all immediately disappear. I've started with the Mac Mini and the MacBook Air both open and watched it happen across both devices.

When I copy back the saved bookmark list, they return to my screens - yet not my iPhone and iPad.

I've had a huge clear-out of old bookmarks that I no longer need, so the list is not an unmanageable length.

iCloud sync is enabled, identically, as per device, across all of my devices, but it just seems to have gone wrong. I'm now noting websites on paper for future reference.