Family sharing changing name- I can change my Wife’s contact card!

My wife and I have separate Apple IDs. We have family sharing for contacts and a few subscriptions.

I wanted to add a funny name in her contact so it comes up, let’s assume it’s ’My Wife’. I put this in as her last name so it comes up.

Her contact card - My Card on her phone- changed to include this.

How is this possible and how can we turn it off?

It does not work in reverse!