Gen-2 Web Component widget - Style Properties

ThingWorx 10.1.0-b47, SDK Utility build matching 10.1.

A custom Gen-2 (Lit-based) Web Component widget, built with the official ThingWorx Web Component SDK Utility (mub).
imports and renders correctly in Mashup Builder, but its Style Properties panel shows zero rows.
The StyleTheme picker is visible, but nothing underneath, not even a flat list, just empty.

We're able to fall back to classic STYLEDEFINITION-based styling, but our goal is the style dictionary tree.

Is there a required steps beyond a normal mub build + import for a Gen-2 widget's style dictionary to show up in Style Properties?