Eu consertei isso excluindo manualmente os dispositivos Raid usando o mdadm antes do início do processo de particionamento.
Do meu arquivo preseed:
# This command is run immediately before the partitioner starts. It may be
# useful to apply dynamic partitioner preseeding that depends on the state
# of the disks (which may not be visible when preseed/early_command runs).
d-i partman/early_command string mdadm --stop /dev/md127 && mdadm --remove /dev/md127