"Unknown Parent" in Family Tree Makes it Impossible to Add New Parents in CAS

Hello,

I'm currently playing with a household where one sim was created by me in CAS and the other was uploaded from the gallery (also created by me but in another save). In the other save, this sim did not have parents assigned. I recently decided to make parents for both of these sims. I had no issues adding the "Mother" and "Father" relationship with the same I created in this current save. However, the other sim (uploaded from gallery/prior save) has one of the parent relationship slots taken up by "Unknown Parent." This makes it so that I can either add a relationship for "Mother" OR "Father" but I can't add both parents via CAS, or MCCC, because the "Unknown Parent" box in the family tree appears to be occupying the place of a real sim, despite not being a real sim. I even tried using "play with genetics" to manually create the two parents, but after I add the relationship with one, the other only gives me the options "Sister" or "Partner" despite no real sim occupying the other parent role.

Has anyone else experienced this or have any tips on how to remove the "Unknown Parent" box? I've read a few people talking about glitches with the new family tree system, but I haven't seen this specific one addressed.

Thanks in advance for any help/advice you can provide on this.

-Cee