I noticed this behaviour:

I am making material for bark, so I drag diffuse, bump, normal texture and it makes 3 gfx files (OK)
Then I make material for leaves. I use same texture (I have atlas with bark and leaves), but now only diffuse texture, because it does not make much sense to have normal maps on small and distant leaves in this case. And result is, that gfx file for diffuse is remade and gfx with normal map is deleted (so material with bark is corrupted).

This is quite annoying, because I have to backup gfx with normal map and after setting up leaves restore it again. Why is engine deleting it? Is there any particular reason for it? This makes importing of assets nightmare.

How does your workflow looks like? Are you keeping whole texture set (diffuse, bump, normal) even in such cases? Is there any performance impact for doing this (I would guess that yes).