The logical failure is in your assumption that the slang for father, Dad, is part of her vernacular. If you want a fat repository of words and their slang equivalence; combined with their Digraph relationships in common vernacular, to their shortest-path results via Djikstra's Networking algorithms, but not have any of this discrete/finite automata stored on your phone, thus force a Cloud storage extension which requires deep search and stored procedures for repetitive and weighted patterns, while also having Apple still not store your data [privacy!!], then you and the rest of your Generation Lost don't understand the technology behind it.
Okay, sure -- I'll buy that "father" --> "dad" is "vernacular" so to speak. However, why then, do both "mom" and "mother" both work to place the call? If the relationship is set up in contacts as such, and it is, it should be able to recognize either. One working and the other not makes no sense. Also, both father and dad used to work until six months ago. So, sounds like a software update broke it?