Is this a bug or intended with family tree?

When the update to the family tree came out I had an existing household comprising of 2 parents and 3 children. I wanted to take advantage of the new family tree update by adding the parents of the 2 existing parents in the household. When I went to select the relationship to the sim however once I locked in the mother it immediately made the father a step father and didn't allow me to select father.

I did have cas.fulleditmode enabled at the time and cc was disabled due to mods being broken. Is this a bug with the family tree or is this intended?