What Family Echo displays for unnamed people depends on how the person was created:
[add child] [add child with new partner] results in "Mother of child of Prescott"
[add partner/ex] [add new parner] results in "Partner of Prescott"
[add partner/ex] [add new ex-parner] results in "Ex-partner of Prescott"
You can correct this by deleting the child, deleting the partner, then adding the partner before the adding the child. You can also correct this by adding the partner's name.