I tried it around the time I originally posted this thread, and that sample code project displays the dialog correctly, but it's not analogous to what I'm discussing here, because it uses IntentDialog. The repo I posted with the reproduction only uses a snippet view without a dialog. The dialog adds a top section with additional information. I just want to display the snippet.
Also, when I wrap a snippet in an IntentDialog, sometimes I get the same behavior where the snippet doesn't adapt to dark mode.
When I tried to build Apple's sample code project again just now, I got this error: Invalid value of WKCompanionAppBundleIdentifier key in WatchKit 2.0 app's Info.plist: com.example.apple-samplecode.AppIntentsSampleApp (expected com.example.apple-samplecode.AppIntentsSampleApp<redacted>)