@crinrict That's a fair point. :-) I guess I just hoped that if it was fixed for one of the reported causes, then maybe all of them were addressed.
After reading through the thread again, the only other common theme appeared to be science babies. So I tested that at the BFF house by having Liberty give birth to a Science baby. She ended up having twins. After age up to infant, she had the full set of interactions, for both infants, without needing to be introduced. Travis and Summer also did not have to be introduced in order to interact with them.
Since there were some who said they had neither beloved trait or science babies, I also tested with the Sigworth family. Jess became pregnant with Dylan and she did not have the beloved trait. Again, Jess had all the normal interactions with no need for introduction.
If the bug is still occurring, then I don't know what might be triggering it. If I see it again, I'll definitely submit a new bug report, as long as someone hasn't already made one.