When importing a mesh, there is an option to Combine Meshes, which by default is checked. Unchecking this option will bring in the individually named parts contained in the FBX as unique objects in the content browser named <filename>_<objectname>.
It is expected that each object that is imported is its own uasset, however it currently imports all of the objects contained in the fbx (uncombined meshes) to one single uasset that contains multiple objects.
Repro:
1. Open 3ds Max, Maya, Modo (will assume 3ds Max for rest of repro)
2. Create a box and name it something.
3. Create another shape (sphere, cylinder, etc) and name it something.
The result here should be two unique objects in the scene, with unique names.
4. Select both objects, and export as FBX (attached)
5. Import the file you just exported to the Content Browser, and ensure Combine Meshes is set to false.
6. Save the meshes
7. Right Click, and Show in Explorer
8. Observe that there is only one uasset created, and it is named the same name as your fbx you imported.
9. Select the two meshes that were imported in the content browser and press CTRL - C.
10. Paste into a text document
11. Observe the last part of the path, should read <fbxfilename>.<fbxfilename><objectname1>, indicating that the objects are contained within the uasset package, instead of the expected name of <fbxfilename><objectname1>.<fbxfilename>_<objectname1>.
StaticMesh'/Game/Developers/maurymountain/Meshes/S_Shapes.S_Shapes_Box'
StaticMesh'/Game/Developers/maurymountain/Meshes/S_Shapes.S_Shapes_Cyl'
If you are to rename any of these meshes, it will correct the packaging, but also not be able to be reimported from the single FBX file.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-28898 in the post.