mapbox-gl production build error "x is not defined"

I upgraded to 6.0.0 (also tried 6.0.1) and everything works fine in development mode, but in the production build the background map doesn't render using. I'm using a mapbox token.

        <StaticMap
          key="map"
          width="100%"
          height="100%"
          mapStyle={MAPBOX_MAP_STYLE}
          mapboxApiAccessToken={MAPBOX_ACCESS_TOKEN}
        />

I have an IconLayer that renders just fine, but on top of a grey background :/

Reproduce the production-only failure with react-map-gl 6.0.0 or 6.0.1 using the shown StaticMap, map style, and Mapbox token, then compare it with development mode. Start by tracing the StaticMap and IconLayer rendering paths; done means the background map renders in the production build instead of remaining grey.