Since I upgraded to High Sierra I’ve been unable to add new contacts to my Contacts list. I start one, begin to enter data, and then it flips to another already completed entry and acts as if I’m trying to edit it.
I talked to a senior support person at Apple today and he told me this is a known problem (along with a couple of other Contacts problems) which is being worked on by Apple programmers. An update will be forthcoming at some point.
So if Contacts is acting flaky for you the recommended procedure is to add new contacts to your iPhone and then sync it with your computer’s Contacts.