The issue seems to be in udev scripts as Pascal Hambourg said. In debian 7 there is one script in intramfs to manage md devices: /lib/udev/rules.d/64-md-raid.rules
In debian 8 there are two different scripts: /lib/udev/rules.d/63-md-raid-arrays.rules /lib/udev/rules.d/64-md-raid-assembly.rules The issue has been introduced here. These files are in package mdadm.
