I have been having this issue for a little bit but I want to solve it. My wife and I both have separate icloud accounts (used to share one when we could), we do family sharing (I am the admin for this). We are having the issue where if I set my contact card, this also sets it as my wifes contact info. If I change it on her device (phone, computer) then it changes my contact info to hers. I haven't been able to figure out the right combination of sharing and accounts to get it to work properly. I recently noticed that I had her icloud account (though disabled) in my accounts, and she had mine. I deleted these and still having the issue. Not sure if this a bug (has been happening over multiple mac and ios versions), or if there is a right combo of settings to get it to work correctly.