I contact Apple yesterday. I had several other issues that seemed related to this one. All involved the wi-fi network in some way. We poked around for a while, not really finding anything. It occurred to me to mention that I run a VPN. Wondered if it could be a factor. The tech support said, "Oh yeah. VPN doesn't play well with iCloud. I disconnected the VPN. One by one I discovered that all the problems that seemed related were resolved.
I've asked the VPN provider why/how a VPN would interfere with so many fundamental processes. Their response was, "We're aware. Working on it." I await an an explanation and a fix. In any case, teh Safari not syncing problem has been fixed.
Also puzzled why I would have had all these problems on my MBA but none of them on my iPhone or iPad, which were also running the VPN.