Lightmaps are generated even if the ISM component's lightmap type is set to Force Volume

1. Create a Blueprint and add an instanced static mesh component.
2. Set an instance mesh, such as a cube, and mobility to static.
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3. Set the lightmap type to Force Volume and create multiple instances.
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4. Place the BP on a level and run a Build Lighing only.

Result:
Check the lightmap in the World settings. The lightmap has been generated.
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If there is only a static mesh and Lightmap type is set to Force Volume, the lightmap will not be generated.
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