We actually had this issue with the STL exporter. STL doesn't have object hierarchy so we must export everything into one big mesh.
See issue here:
https://github.com/SketchUp/sketchup-stl/issues/129
Then I think the commit that fixed was this one:
https://github.com/SketchUp/sketchup-stl/commit/4ad199a2d46245f2a668e47f2ee81bc36c9b9763