The exporter export the render data and the nanite render data is on the gpu and is not accessible, this is why it export the fallback mesh. Its more complex then it appear since we will need to do something similar to what you are doing to export it, we have to build the raw nanite mesh with the LOD build options to have the same result you see in the viewport.
The current workaround is to disable nanite on the asset, export it, re-enable nanite
Export a nanite enabled static mesh
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-172208 in the post.
|UE - Editor - Content Pipeline - Import and Export
|Dec 7, 2022
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|Sep 15, 2023