iPhone Calendar no longer syncs with Mac & vice versa (local syncing – not iCloud)

I have been syncing calendars between my iPhone and my Macbook Pro peer-to-peer (i.e. not using iCloud) for years. These are not Outlook calendars or anything server/network related, just locally created calendars.

Since I upgraded to OS X Ventura, calendars no longer sync between the devices. The syncing process seems to go through the motions, but the calendars on either the iPhone or the Mac don't update with each other's events. (Music files seem to sync OK).

In addition to this, after applying the security update to OS X 13.4 last week, a new message started appearing in the Finder iPhone window under 'Info' saying "iPhone is syncing Calendars over the air" and "Over-the-air sync settings can be changed on your iPhone."

I've looked on my phone, but I can't see anywhere that I can change these mysterious settings. I've no idea what it means, but it's not something that I have changed manually. I've tried creating new calendars on the phone and Mac, but these aren't recognised across the devices either.

Just to clarify, I don't use iCloud and never have done and I don't want to, (so please no iCloud related suggestions). I just want to be able to sync my calendar events locally between the Mac and iPhone like I've been doing for years.

Any suggestions for getting this working again would be great.

Thanks!