will confirm the drawRange issue then report on three js github (made a thread to discuss this)
update:- confirmed that gltfExported does not export drawRange and there are no plans to support it in the future
here’s how the meshes currently export , i think the geometry.groups case has not been fully implemented in GltfExporter
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Update
Made a custom solution , working good so far
sandbox is here bvh csg export - CodeSandbox
method breakdown
- clone the evaluator result geometry
- splice the
geometry.groupsarray so it contains only one material’s groups in each geometry - use
mergeGroupsfromBufferGeometryUtilsto get the new geometry - make a new mesh with the new geometry correct material
these new meshes exports as it looks in gltf 
amazing work with the library! even with all this extra logic it’s still runs smoothly/realtime ! 