My wife and I steal each others contact info
This is both MacOS and iOS, probably an iCloud thing.
Currently, my phone (and MacOS account) think I'm my wife. If I do something with Siri, she calls me by my wife's name and if I try to enter info in a web page (name, email, etc.), it is my wife's information that can be autofilled.
I know I can switch this under iOS under settings -> Siri & Seach -> My information or my contact info as "My Card" under Contacts on MacOS seems to do the same thing. If I switch using either method, then my wife's phone this that she is me.
We each have separate apple IDs that are under family sharing. We do both share a contact list under separate (not hers or mine) Apple ID.
This has been going on a few months (pre- iOS 13). We both have our phones up to date and MacOS is on Catalina.