My MacBook Air notes abruptly stopped syncing with iCloud

MacBook Air M1, 2020 running OS Sequoia 15.1.1 (24B91). I noticed that Notes stopped syncing with my iPhone. I logged onto iCloud and found that iPhone and iCloud were in sync but Notes on iCloud were out of date from Notes on Mac by only a couple of hours. Changes made there reflected on iPhone but not Mac and vice versa. On System Settings>General>Apple Account>iCloud>Notes “Sync this Mac” was On. I turned it off and then back on. No help. I restarted the Mac. All my Notes disappeared (!!). They soon returned one by one, but in the out-of-date version on iCloud. Even so, further changes to Notes on Mac or iCloud are not reflected on the other. This all happened within the past 24 hours. Yesterday it was working normally.

[Re-Titled by Moderator]