I suspect that the installer sees the correct disk as /dev/sdb but doesn't show the mapped partitions yet.
The naming scheme can be messy sometimes and lead to wrong assumptions and choices.
For one, /dev/mapper entries wouldn't be the real device name but those created by the volume mapper and are probably inside /dev/sdb...... that can be confusing as after all, they're not physical partitions .
I myself have never worked with fake-raid (only encrypted LVM partitions), so I'm not sure and thus don't want to advise into simply carrying on (although I suspect you can) ..... unless you've made backups of everything you want to keep and know how to put them back.
- Having recent backups is always good.
This is more of a thing @Thanatermesis should answer ..... I'm fairly sure he has something in place inside the installer that recognizes fake-raid.
I made a short howto on how to access encrypted LVM partitions which can shed some light as to how this kind of mapping works:
https://forum.elivelinux.org/t/resetting-password-using-chroot-on-encrypted-lvm-filesystem
