Sorting layer issues with materials

In a 2d game:
1) I place my background sprite at the coordinates (0,0,0) and set sorting layer to background
2) I place another sprite at the coordiantes (0,0,0) and set sorting layer to foreground
-> Everything is fine, I see my second sprite in front of the background
3) I change the material of my second sprite.
-> I don't see my second sprite anymore although nothing else changed

Can somebody explain this to me?