Why does the 20.04 installer think that a plain linux raid is made up of 2 x /dev/sdb2 partitions?

I've been trying to figure out this one for a few evenings now. I downloaded the 20.04 server installer, and tried to install it on a linux raid which I configured in the installer. This consistently failed.

I then jumped into a shell just after the installer started up, and manually created a linux raid (two disks / mirrored). The partitioning table of both /dev/sda and /dev/sdb is a grub / boot partition on /dev/sdX1 and a /dev/sdX2 partition which takes part in /dev/md0.

Once the linux raid is up and running, i jump back to the installer and try to install Ubuntu on the existing raid. This fails as well.

The error I get is this :

comparing device lists : expected ['/dev/sdb2', '/dev/sdb2'] found: ['/dev/sdb2', '/dev/sda2']

I don't get that! Why does the installer expect that the linux raid should be made up of 2 x /dev/sdb2?

During some attempts, I've also seen expected ['/dev/sda2', '/dev/sda2']. It appears that the installer "randomly" picks a single partition that it expects to see twice - but that's just an observation (or speculation).

No matter how I try to install Ubuntu 20.04 on a linux raid, it fails. A regular install goes just fine.