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