FBX reimport material errors

Apologies if this is well documented elsewhere but I can’t seem to find a definitive solution - When I import a model (with a single multi-sub material applied - 13 used slots but only 6 of them are relevant to the exported model) that has come from 3DS max in the FBX format, it works ok if I combine meshes on import. If I don’t combine meshes the material assignment is wrong but I can combine so that’s not an issue really. It’s worth noting only the 6 used materials are imported, with the others ignored. If I then re-export with a second model included in the export, again using the same multi-sub material, after I re-import the model into UE4, the additional materials don’t get imported and so the second model has the wrong textures applied to it, leaving me with the same original 6 materials. If I delete the assets from the project and import the model afresh, I get all 13 materials no problem but obviously this isn’t an ideal process and would prefer assets to reimport correctly if changed.

I tried using OBJ but found UE4 creates duplicate materials for every model that uses the same material but again reimports in the same way as FBX described above so that doesn’t help either.

Can anyone offer any advice or point me in the direction of a stable export/import process from 3DS max.

Thanks in advance.

If you have a material in a multi/sub-object material and it is not used, then it won’t be imported in UE4, it will reduce the number of materials to only the materials that are actually assigned to some portion of the mesh.