I had a similard behaviour due to upgrade from Jessie 8.0 to 8.5 straight ?
Only my /home is a RAID , I booted without the /home , then could assemble manualy ,ie "madam -A -scan " then run. According to "https://superuser.com/questions/287462/how-can-i-make-mdadm-auto-assemble-raid-after-each-boot"; I edited the *|/etc/init.d/mdadm-raid|* file and inserted the line *|partprobe|* after the header |/etc/init.d/mdadm-raid belongs to mdadm 3.4-1 |*| |* In the old days, we split the swap-space on the beginning of each disk. So my raid is still built with a partition sdx2 instead of whole disk sdx. So i still need partprobe at that place. I can't tell if the bug arise from new udev or systemd. Fixed for me
