Debian's partitioner is really buggy and a major pain point for me. Won't support a backup EFI partition, creating and booting from RAID volumes is really tricky, many other issues. Issues that are resolved or nonexistent in RedHat, Ubuntu, and even Windows. I've had it create unbootable GRUB configurations many times - it only tells you at the very end.

I now make the partitions beforehand in fdisk then choose them in the installer. Which for some bizarre reason is not included in the install image; you have to boot into rescue for that.