"Related People" Field in Contacts (MacOS) not syncing with iCloud

This problem has just started happening in recent weeks. I have created a custom name for 'Related People / Related Name" in Contacts on the computer . . . the name is "children". I put the children's name in the data field. Everything shows up fine in my Contacts on my Mac:

children

Peter • John • Sally (etc)

But the above is not syncing to iCloud, and ultimately, not syncing to my iPhone. For years, I've had no issue with it syncing from my MacOS Contacts all the way to my phone.

If I enter the "children" on my iPhone, it syncs with iCloud (or vice versa). So there appears to be an issue with the MacOS Contacts in the Related People field.