Some Sims Not Showing Up Married In Family Tree or Relationship Panel

I've just noticed that a couple of my Sims are not showing up as married to their partners in The Sims 4. They don't appear to be married in the family tree or the Relationship Panel. It doesn't show that they recognize in-laws or stepchildren. In fact, it doesn't show that they are even in a relationship at all, but the friend and romance bar are full to the max! I noticed that other Sims display their marriage status and appear as normal in their family tree and relationship panel.  So far, I see that at least two families do not show up as married or in a relationship.

As I type this... I wonder if it could be that Sim(A) is married to her childhood friend(SimB), and SimB's sister (SimC) is married to SimA's uncle(SimD), but they have been married before they even became a couple, and they have been together since Sim A&B were kids. The other Sim that doesn't show up as married or in a relationship with her partner is the sister of SimA (We'll call her SimE), and they share the same uncle(SimD), but her husband(SimF) has no relation to either party, so I don't see how Sim E&F relationship would be broken. Could this be a bug, or is it a common problem that could be created through marriage and in-laws?