Calendar not syncing alert times correctly (and alerts not sounding)

Since the most recent OS/IOS updates, Calendar no longer syncs alerts to my iPhone correctly.

No matter what time I set the alert for on my iMac (usually 2 hours before start), once synced to my iPhone, it is on my iPhone at 'At Day Of Event (00.00)', which is no use to anybody.

I sync manually via cable, not via iCloud.

In addition, the alerts no longer sound on either my iMac or my iPhone. Notifications are all set to 'on', the volume is turned up and Do Not Disturb is not active.

Previously it has worked perfectly for years and I haven't changed any of the settings.