fstab entry not being unmounted during shutdown

My exfat filesystem is not being umount'd properly during shutdown/reboot.

Here is my fstab:
# Device     Mountpoint FStype Options Dump Pass#
/dev/ada0s3a /          ufs    rw      1    1
/dev/ada0s3b none       swap   sw      0    0
/dev/ada0s5  /archive   none   rw,mountprog=/usr/local/sbin/mount.exfat 0 0
It mounts properly during boot, but when I reboot, I get this message:

Dec  6 22:23:58 cinofbsd kernel: Starting file system checks:
Dec  6 22:23:58 cinofbsd kernel: /dev/ada0s3a: FILE SYSTEM CLEAN; SKIPPING CHECKS
Dec  6 22:23:58 cinofbsd kernel: /dev/ada0s3a: clean, 59234518 free (4022 frags, 7403812 blocks, 0.0% fragmentation)
Dec  6 22:23:58 cinofbsd kernel: Mounting local filesystems:
Dec  6 22:23:58 cinofbsd kernel: FUSE exfat 1.2.4
Dec  6 22:23:58 cinofbsd kernel: WARN: volume was not unmounted cleanly.
Dec  6 22:23:58 cinofbsd kernel: .
Note how the first two filesystems are CLEAN, yet the exfat fs is not unmounted properly during shutdown.

Any ideas as to how I can make FreeBSD unmount this fs properly?

EDIT: Also note that if I manually umount the fs before rebooting, the warning does not appear. Also, 'umount -a' unmounts the exfat fs, so for some reason the shutdown scripts are not recognizing my exfat fs as something to umount.
On fstype you have none, can that be msdosfs, instead of mountprog and exfat? Shouldn't you have 2 and 2 for Dump and Pass?

If your documentation says those settings are right, maybe it's that program's settings in /usr/local/etc/.
msdosfs does not handle ExFAT filesystems, for instance:
# mount -t msdosfs /dev/ada0s5 /mnt
mount_msdosfs: /dev/ada0s5: Invalid argument
Also, if I put 2 in for Pass# in fstab the system fails to boot because there is no command called fsck_none (it's trying to call fsck on the type, in this case none).
# pkg info -l fusefs-exfat
The package does not provide a fsck program.

So, the only solution for now is to umount the filesystem in rc.shutdown (though I kind of think it would be sensible for FreeBSD to do this by default, but oh well)
To clarify, umount -a does unmount the exfat FS; it's just that rc.shutdown does not call umount -a or anything to that effect, but it appears to be fine if I just add it in manually.

The only lingering thought is why FreeBSD does not do that by default.