ZFS missing space

I've noticed space "go missing" on ZFS before, but not conclusively. 
This time it's happened on a brand new setup I'm doing some testing on.

This is a clean install of 14.2. The machine rebooted (not sure why) 
while a large dataset was being received (zfs receive...). The dataset 
isn't there.

There should be about 6Tb in the pool (depending on how you count a Tb).

This is what I'm seeing as varying estimates of free space:

root@zfs2:/home/fjl # zfs list -t all
NAME              USED  AVAIL  REFER  MOUNTPOINT
zr               30.6G  5.12T   140K  /zr
zr/ROOT          5.12G  5.12T   140K  none
zr/ROOT/default  5.12G  5.12T  5.12G  /
zr/data           140K  5.12T   140K  /data
zr/francis       23.3G  5.12T  23.3G  /zr/francis
zr/home           343K  5.12T   140K  /home
zr/home/fjl       203K  5.12T   203K  /home/fjl
zr/tmp            140K  5.12T   140K  /tmp
zr/usr           2.19G  5.12T   140K  /usr
zr/usr/ports     1.16G  5.12T  1.16G  /usr/ports
zr/usr/src       1.03G  5.12T  1.03G  /usr/src
zr/var           1.11M  5.12T   140K  /var
zr/var/audit      140K  5.12T   140K  /var/audit
zr/var/crash      140K  5.12T   140K  /var/crash
zr/var/log        401K  5.12T   401K  /var/log
zr/var/mail       180K  5.12T   180K  /var/mail
zr/var/tmp        140K  5.12T   140K  /var/tmp
root@zfs2:/home/fjl # zpool iostat
               capacity     operations     bandwidth
pool        alloc   free   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
zr          42.8G  7.22T      7      8   417K  1.16M

As this is a "clean" system I can't figure out where the discrepancy 
could possibly be coming from. This is beyond the slop value.

Any ideas anyone?

(Incidentally, I'm testing failure modes using some flaky hard drives I 
acquired on eBay - no errors logged prior to reboot).

Thanks, Frank.