After reloading Sonoma and then restoring my home files (per ACE’s suggestions), it seems that Reminders is totally broken. I can’t create a reminder, let alone get anything to sync from the phone. Calendar entries don’t sync, either. I’ve disabled iCloud for that, because of other problems. It “Used to Just Work” that I could directly sync calendar and reminders between the phone and the Mac.
(And as a side comment, I’ve given up on calling Apple support for anything remotely related to iCloud or syncing. Just not worth my time, because the tier 1 people suggest stuff I’ve already tried, and the tier 2 people have never followed through when they couldn’t fix stuff. That’s a HUGE change this year from my prior experience with Apple support.)
I seem to remember that at various times, Apple has updated the database formats used for Notes and Reminders (and possibly other common apps). When they did this, you would be asked to upgrade your existing content. If you say yes, it will become incompatible with older apps that don’t know about the new format (e.g. other Apple devices not running the latest system software, possibly because they’re incompatible or maybe just because you don’t want to upgrade them).
I know I’ve refused several such database updates because my laptop is a 2011 MacBook Air running Sierra, and I don’t want to lose the ability to access these apps’ iCloud-hosted data. I’ll probably upgrade them some time next year when the lack of ongoing Firefox support will finally force me to upgrade this computer.
Unfortunately, once you choose to upgrade these documents, I don’t think it is possible to go back
