Since the recent change of systemd, grub-once isn't called any longer, thus at
thawing, the normal GRUB menu is shown instead of the direct boot to the target
kernel.

The regression was introduced in openSUSE:Factory/systemd changeset 215:

Wed Feb 18 05:01:38 UTC 2015 - crrodriguez@opensuse.org
....
- systemd-sleep-grub: moved to the grub2 package where it belongs as a
  suspend/resume hook (SR#286533) (drops prepare-suspend-to-disk.patch)

However, SR#286533 was declined and it's lost since then.

The fix would be either revert the change in systemd or put the hook properly
into grub2 package.

The latter method has a potential bug, however.  systemd-sleep doesn't call
hooks to restore when writing to /sys/power/state fails.  That is, if we have a
hook to set up grubenv, it'll left when the hibernation fails, and the next
boot would go wrong.  This didn't happen in the past because it invoked the
grubenv setup in a different manner.