For kicks, I renamed ~/Library/Calendars, saw macOS’ empty Calendars, and did a Finder’s iPhone sync. Bad idea. iPhone’s calendar events got removed too. DOH! So, I exited macOS’ Calendar, put back the old ~/Library/Calendars, rechecked macOS’ Calendar, old datas were there without the newer datas (added events), resync, and of course iPhone don’t have all the datas like the new added events. I made a new calendar event as test for all day and then resync, and it showed up in macOS’ Calendar. Deleting iPhone’s test all day calendar event and resynced. Both test event gone. Hmm!
Apple needs to provide a way to go from iPhone to macOS only when syncing (Finder and iTunes have the option to from macOS to iPhone to overwrite). Dang it, Apple. Come on please. :(