Well, it's at least good to know that it's not just me. Misery loves company…
This happened to me a few months ago, and I called Apple support, and got up to a higher level of support. I finally had to go back to the solution that one of the earlier levels had given me, which was to turn Siri off and on.
That was the only thing that work. And it did work, but it seemed like I had lost some of this Siri training that had accumulated.
So now, after another few weeks of training, and dictating, this problem has cropped up again. And there seems to be nothing that I can do to fix it. I tried to turn Siri off and then on again solution, I tried a reset all settings, I tried turning on and off the phone, and a few others among the usual suspects of troubleshooting steps.
Nada. Zilch. Nothing.
And I think that the geek's diagnosis is probably correct. Hopefully it will be fixed soon. Perhaps even before the new update.