I didn't know about this "feature", but to be honest I have learned years ago not to use the partitioning features of the debian installer. It's just another crude tool to fight with, while the real tools are just a console away.

Here's a list of the things the debian-installer can't do, or at least couldn't do when I needed it:

- accurate partitioning, such as choosing between GPT or MBR partitioning

- filesystem features (btrfs subvolumes, ext4 flags)

- creation of crypt volumes with keyfiles

- having user control over the names of crypt devices

- using an existing crypt device WITHOUT formatting it

- mdadm write-intent bitmap enabling/disabling