Steps to reproduce :
* Import attached fbx
* Select in the project view
* Click the Materials tab in the Inspector Window
* Open the Material Selector for the "No Name" material
* Select a different material, see that the material changes
* Select a second different material, see that the material does NOT change
* Further changes will not update the material

Workaround:
* Close the Material selector
* Open the Material Selector
* The second selector works as expected

The same issue is occuring with some material selectors in the particle system editor