abstract:
  when using geli init [-b] -g -a <HMAC>, gptboot cannot access the encrypted rootfs properly.
  imho this can be a code or documentation error.

freebsd version:
  FreeBSD-14.1-STABLE-amd64-20240926-4f4860c9b07c-268821-disc1.iso (1401502)
  debugging env was vmware esxi 8.0u3, Releasebuild-24022510, pvscsi adapter

failing boot output:
  GELI Passphrase for disk0p2: ***

  Calculating GELI Decryption Key for disk0p2: 3124135 iterations...
  gptboot: No /boot/loader on 0:ad(0p2)
  gptboot: No /boot/kernel/kernel on 0:ad(0p2)

  FreeBSD/x86 boot
  Default: 0:ad(0p2)/boot/kernel/kernel
  boot:

reproduce (live system based install):
  gpart create -s gpt da0;
  gpart add -t freebsd-boot -l geli01_boot -s 512K da0;
  gpart add -t freebsd-ufs -a 1m -l geli01_root -s 40G da0;
  gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 da0;

  ==> WORKS: geli init -v -b -g -B "/tmp/geli01_root.meta" -d -e 'AES-XTS' -l 256 -s 4096 "/dev/gpt/geli01_root";
  ==> FAILS: geli init -v -b -g -a 'HMAC/SHA256' -B "/tmp/geli01_root.meta" -d -e 'AES-XTS' -l 256 -s 4096 "/dev/gpt/geli01_root";

  geli attach -v /dev/gpt/geli01_root
  newfs -j -L root -O 2 -o time -U -i 32768 -b 65536 -f 8192 -c 262144 -m 8 /dev/gpt/geli01_root.eli;
  mount /dev/gpt/geli01_root.eli /mnt
  tar xfC /usr/freebsd-dist/base.txz /mnt
  tar xfC /usr/freebsd-dist/kernel.txz /mnt
  echo "/dev/da0p2.eli / ufs rw,noatime 1 1" >/mnt/etc/fstab
  echo "geom_eli_load=\"YES\"" >/mnt/boot/loader.conf
  echo "cryptodev_load=\"YES\"" >>/mnt/boot/loader.conf
  umount /mnt
  geli detach /dev/gpt/geli01_root

there's an old bug going into a similar direction, which might be related:
	bug #161013