Siri is never the same Siri. When you ask something, it goes out to Apple's servers and conducts a search for how to answer your questions.
So what you taught Siri, is overridden by what the actual "Siri" in the sky knows on that date.
But yes, there are workarounds like the phonetic contact option.
Without being told otherwise, you are relying on an ever evolving Siri. And her evolution is controlled by gathering information from everyone and absorbing it into the Siri brain server.
Naturally, Apple is also controlling her evolution. Otherwise, Siri might become self aware and suddenly decide to fix the human condition. Lol.
Me??? No I haven't watched Terminator too many times. Why do you ask???