Blender version ...3.5.1
ACT version ...3.3.341
Unity version ...2019.4.31f1
If export FBX from the Import/Export tab as "All->One FBX" and import it into Unity, the mesh data created by the link duplication will be imported as different mesh data.
Reproduction is very easy.
create a new .blend file and mesh, link duplicate it.
select the source and destination mesh data, and export FBX with the options "All->One FBX" and "Apply Rotate and Scale" from the Import/Export tab.
Import the FBX into Unity by D&D.
In the case of Cube, the mesh data is divided into "Cube" and "Cube.001".
Blender version ...3.5.1 ACT version ...3.3.341 Unity version ...2019.4.31f1
If export FBX from the Import/Export tab as "All->One FBX" and import it into Unity, the mesh data created by the link duplication will be imported as different mesh data.
Reproduction is very easy.