When my friend syncs their iPhone over USB with their Mac, contacts and calendar entries they create on their Mac are not copied to their iPhone, but contacts and calendar entries they create on their iPhone do get copied to their Mac. How can we get this data syncing bidirectionally again?

This data used to sync in both directions correctly, but stopped doing so around the time they updated to iOS 14.4.2 (it may very well have started before the update; they first noticed about a day after updating). It's hard to tell when exactly it started exhibiting this behavior because there’s no error message when they sync. The behavior we do observe is that once the sync gets to the calendar step, it sits there indefinitely until the sync is cancelled. If we turn off calendar sync, it gets to the contacts step and similarly sits there indefinitely until the sync is cancelled.

They do not use iCloud. They're running macOS Big Sur 11.2.3 and sync using the Finder. They use Apple’s Contacts and Calendar apps on both their phone and computer. "Sync contacts onto iPhone", Sync "All Groups" is selected, and "Sync Calendars onto iPhone", Sync "All Calendars" is selected.

We've tried booting the Mac into safe mode, but this appears to completely disable whatever Finder component handles iOS syncing. We also tried performing a sync while streaming the AMPDevicesAgent logs, but when the sync hangs, the logs just show a bunch of "sending Ping for device" and "got Ping message for device" messages with no errors until we cancel or disconnect the phone.

Something strange we noticed that may or may not be relevant is that in the Finder sync settings, the box for “Add new contacts from this phone to ‘ALC Board’” is checked (this is one of their groups in Contacts) even though they never chose it. However, when they try to pick a different group for the destination, it only shows a list containing their other groups but not the All Contacts folder. If they uncheck this box, and click Apply, after syncing the box comes back checked (sometimes a different group name will appear). I am unsure whether this is relevant because they can sync from their phone to their Mac.

Similarly strange, in the settings from Calendar sync, the ‘Do not sync events older that — days’ is currently unchecked. However, originally it was checked and set to 730 days. When they set it to 0, it was supposed to transfer all calendar entries from Mac to phone, but it did not (and did not show an error message).

Thank you for your help!


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