*** RESOLVED ***
Here's what I did:
1. Disabled Siri on the lock screen (Settings -> TouchID and Passcode -> scroll to allow access when locked and slide "Siri" to off.)
2. Disabled Hey Siri. (Settings -> General -> Siri, slide "Allow Hey Siri" to off.)
3. Disabled Siri. (Not impressed...) (Settings -> Genral -> Siri, slide "Siri" to off.)
4. Restart Phone.
5. Re-enable in reverse order (Enable Siri, enable Hey Siri, enable Siri on the lock screen.)
It works now. I will say that I find I have to nearly scream at it to wake it up. Seems worse when the screen has gone into standby. You can usually shout your whole command immediately, but if you prefer to save your breath, say "Hey Siri" first and wait for the tone.
Should anyone from Apple be paying attention, the ability to "re-train" the Hey Siri recognition would be great. Better recognition and a way to differ the phrase, like "Yo Siri" or "Oh Siri..." might be cute, among other things people would undoubtedly say.......
Alright. Good luck. I imagine iterative improvements for this non-well-documented feature will eventually be forthcoming.