Comment 24 for bug 577239

@Kradllit
From the bugtracker you can see that ubuntus modifications to the raid package have been unsupported for years.
https://wiki.ubuntu.com/ReliableRaid

Ubuntus mdadm setup is in the general case not capable to degrade gracefully on boot. Instead of fixing the disabled notification of the admin/user in case of a failure, there was even a switch added to officially prevent the booting and graceful degration to happen automatically, as if that would have been possible in other than a most simple setup.

Cryptsetup making sure to open only a mapper device created by the raid system seems to have saved you from Bug #531240.