After spending an hour with Apple Support, I learned that Apple has removed some Siri functionality. My wife and I both own Apple Watches (Series 3) and iPhone Xs.
Previous behaviors on WatchOS: “Siri add item to my groceries list in Reminders”… Such a command to Siri would result in said item being added to the named list in the Reminders.app. Under the current WatchOS updates [4.3.1 (15T567) & 4.3.2 (15U70)] Siri reports that no such list exists and prompts to go to the iPhone. My very supportive Apple rep confirmed this after much testing and consultation.
Given the form factor of the Apple Watch, this is a problem for us and a significant step backward. Siri is a major interface to the Watch. I requested through the Apple Feedback page that they please return this much-needed functionality. My hope is that this is a momentary lapse as Apple prepares for their big changes to Siri in the upcoming months. I share this with the Mac Power Users community that they will be saved an hour on support and hopefully help in advocating the keeping of such a feature.