For anyone still interested in this.
Unreal will assign material slots in a ranking order that mimics the meshes ranking order in your 3D modelling program.
Example:
if I have a mesh containing 2 cubes crashed together.
My outliner in Maya may look like this:
Cube_01
Cube_02
Thus whichever material is assigned to Cube_01 will appear in Material slot 1
If the order of these changes during edits and the outliner looks like this:
Cube_02
Cube_01
if I reimport from this state then it will appear as if the materials have swapped position in Unreal
So just be aware of how things are ordered in your scene before export, as this will directly affect material slot ranking on import.
If you have a single mesh with multiple materials on in and wish to change material slot order.
The easiest way I have found if to just break the mesh apart and make sure the corresponding faces are in the order you want.