Static meshes lose their static mesh reference when alt-dragged in viewport. This only occurs if the mesh is located in a filepath containing square brackets (  ) within the filepath name. Additionally, the mesh cannot be reset to any mesh located in the filepath with  in the name within the details pane, but can be set to any other mesh.
1. Download and open attached .zip
2. In viewport, select cube closest to the viewport camera.
3. Alt+LMB transform widget and drag to create a copy of the asset
Mesh disappears, reference to asset remains but the static mesh in "Details" pane is set to "none". Cannot be set back to "Cube", but can be set to any other mesh.
Mesh does not disappear upon alt+drag, a copy of the mesh is created at the new location in the viewport. Static Mesh section of "Details" pane is still populated and can be set to any mesh, including "cube".
Head over to the existing Questions & Answers thread and let us know what's up.