Mapbox GL does not AA polygons

I have a Mapbox style with Filled Polygon layers, with AA enabled for these layers.

The layers look correct on Mapbox Studio (Mapbox GL JS). Under Mapbox GL on Qt 5.9 all polygon layers are aliased. They have the same appearance as when I uncheck "Antialias" for the layer in Mapbox Studio.

See the attached images comparing the same region under Qt vs Mapbox Studio.

In case it is style-specific (I doubt it) the style URL is mapbox://styles/drivecxtest/cj3ubudw5002d2rmuc9avde7f

Note that Line layers render properly anti-aliased on the same style. So, AA does work in general in the map, just not on polygons.