I recently upgraded to macOS 10.13.3 and iTunes 12.7.3 thinking Apple should have fixed several bugs by now. I connected my iPhone 6s with the USB cable and was shocked to find that Contacts wouldn't sync at all, even when I attempted to replace all Contacts on iPhone from AddressBook. After a long an painful discovery of trying to debug Apple's software (which should work seamlessly with supported Apple products). I had to revert back to OS X 10.11.6 and iTunes 184.108.40.206 in order to properly sync Contacts again. #(*$&% !!! I'm appalled that Apple would even release software without even testing this basic fundamental feature, which should have been caught in regression tests. I have confirmed it is not iOS that has the problem. The root cause I suspect would appear to be that Apple keeps changing the main database architecture and perhaps other departments didn't get the memo. This issue seems to be a hot topic as it's been reported in several cases all over the internet in different languages. I plan to remain on 10.11.6 until Apple gets there #($*%* together and starts releasing software with some sort of testing. This is pathetic! I hopes this post helps others to find a working solution more quickly that it took me, going back and testing 10.13.3 ->10.13.2 -> 10.13.1 -> 10.12.x -> 10.11.6.