This behaviour was confirmed on Sony Z5. For a very long time we were able to read unique SD Card serial number by accessing contents of files under /sys folder. Now it is restricted.
There is a Stack Overflow question on this topic here: http://stackoverflow.com/questions/43484564/how-can-i-retrieve-an-sd-cards-serial-number-in-android-..., and it appears as intentional move.
I am trying to get answer from Google if that is Their recomendation, or maybe that is what OEMs are allowed to do since Nougat.
