What you are experiencing that generally happens when you have assigned both the materials to the same inspection plan (same group). Kindly check the material assignment at header level in QP02.
If you want to maintain 2 separate values for each materials then either you should delete one of the materials' assignment from QP02 and create new inspection plan for that material OR you should make use of dependent characteristic in the same inspection plan where you can make material specific exception for MIC values.