The MultiMesh asset works which got me looking into this further. I reduced a project down to two objects and still have the re-import issue. I then renamed the individual meshes in shorter before exporting and it fixed the case. Please look at the name differences to see what the specific cause in the name is:
Not Working Mesh Name : NotWorking_M_Concrete-Rectangular-Column_750x600mm__1137538_
Working Mesh Name : Working_Column1
Working and Not Working FBX Files
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Leigh