holes in multipolygons are lost by simplification

[mephinet]

I'm experiencing an issue when simplifying a geometry that has a hole in it (and is thus a Multipolygon) in the input). This reproducer downloads a Geopackage, extracts a single Multipolygon, and then uses topojson to simplify it. It saves a GEOJSON before and after simplifying. You can see that while the original GeoJSON still has a hole, the simplified one does not any more: https://github.com/mephinet/topojson-reproducer My first guess is that the nesting level in the "coordinates" confuses topojson..?