ZFS datasets no longer automatically mount on reboot after system upgrade

I have an Arch linux ZFS backup system which wasn't updated for a long time because it was working perfectly and there was no reason to bother. After a recent upgrade, though, my ZFS datasets are no longer automatically mounted when I reboot. Unfortunately, the services don't indicate any error messages, and I can run

# zfs mount -a

after the system is booted with no problem. Some particulars:

[root@elephant etc]# uname -a
Linux elephant 4.16.11-1-ARCH #1 SMP PREEMPT Tue May 22 21:40:27 UTC 2018 x86_64 GNU/Linux
[root@elephant etc]# pacman -Q | grep spl
spl-dkms 0.7.9-1
spl-utils 0.7.9-1
[root@elephant etc]# pacman -Q | grep zfs
zfs-dkms 0.7.9-1
zfs-utils 0.7.9-1

root@elephant etc]# zfs get mountpoint backup/www
NAME                              PROPERTY    VALUE                              SOURCE
backup/www  mountpoint  /backup/www  default
[root@elephant etc]# zfs get mountpoint backup/data
NAME                               PROPERTY    VALUE                               SOURCE
backup/data  mountpoint  /backup/data  default
[root@elephant etc]# zfs get mountpoint backup/metadata
NAME             PROPERTY    VALUE             SOURCE
backup/metadata  mountpoint  /backup/metadata  default

Notice in particular that the zfs-mount service seems to be perfectly happy:

[root@elephant ~]# systemctl -l status zfs*
● zfs.target - ZFS startup target
   Loaded: loaded (/usr/lib/systemd/system/zfs.target; enabled; vendor preset: enabled)
   Active: active since Mon 2018-05-28 15:30:18 CDT; 1min 32s ago

May 28 15:30:18 elephant systemd[1]: Reached target ZFS startup target.

● zfs-import-cache.service - Import ZFS pools by cache file
   Loaded: loaded (/usr/lib/systemd/system/zfs-import-cache.service; enabled; vendor preset: enabled)
   Active: active (exited) since Mon 2018-05-28 15:30:18 CDT; 1min 32s ago
  Process: 659 ExecStart=/usr/bin/zpool import -c /etc/zfs/zpool.cache -aN (code=exited, status=0/SUCCESS)
  Process: 656 ExecStartPre=/sbin/modprobe zfs (code=exited, status=0/SUCCESS)
 Main PID: 659 (code=exited, status=0/SUCCESS)

May 28 15:30:12 elephant systemd[1]: Starting Import ZFS pools by cache file...
May 28 15:30:18 elephant systemd[1]: Started Import ZFS pools by cache file.

● zfs-zed.service - ZFS Event Daemon (zed)
   Loaded: loaded (/usr/lib/systemd/system/zfs-zed.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2018-05-28 15:30:18 CDT; 1min 32s ago
     Docs: man:zed(8)
 Main PID: 1586 (zed)
    Tasks: 3 (limit: 4915)
   Memory: 5.6M
   CGroup: /system.slice/zfs-zed.service
           └─1586 /usr/bin/zed -F

May 28 15:30:18 elephant systemd[1]: Started ZFS Event Daemon (zed).
May 28 15:30:18 elephant zed[1586]: ZFS Event Daemon 0.7.9-1 (PID 1586)
May 28 15:30:18 elephant zed[1586]: Processing events since eid=0
May 28 15:30:18 elephant zed[1591]: eid=1 class=history_event pool_guid=0x7314E37F1A1C0088
May 28 15:30:18 elephant zed[1593]: eid=2 class=config_sync pool_guid=0x7314E37F1A1C0088
May 28 15:30:18 elephant zed[1595]: eid=3 class=pool_import pool_guid=0x7314E37F1A1C0088
May 28 15:30:18 elephant zed[1618]: eid=5 class=config_sync pool_guid=0x7314E37F1A1C0088

● zfs-mount.service - Mount ZFS filesystems
   Loaded: loaded (/usr/lib/systemd/system/zfs-mount.service; enabled; vendor preset: enabled)
   Active: active (exited) since Mon 2018-05-28 15:30:12 CDT; 1min 38s ago
  Process: 657 ExecStart=/usr/bin/zfs mount -a (code=exited, status=0/SUCCESS)
 Main PID: 657 (code=exited, status=0/SUCCESS)

May 28 15:30:12 elephant systemd[1]: Starting Mount ZFS filesystems...
May 28 15:30:12 elephant systemd[1]: Started Mount ZFS filesystems.
[root@elephant ~]#

However:

[root@elephant ~]# zfs mount
[root@elephant ~]# 
[root@elephant ~]# zfs mount -a
[root@elephant ~]# zfs mount
backup                          /backup
backup/data  /backup/data
backup/metadata                 /backup/metadata
backup/www  /backup/www

I've run out of time available to deal with this and am just going to switch all the datasets over to legacy mount, but I'm still curious why this worked for zfs 0.6.x but isn't working for 0.7.9 -- possibly some kind of systemd incompatibility?