FreeBSD 11.2-RELEASE, *based on mfsbsd,
*not* tested on *vanilla* FreeBSD 11.2-RELEASE*.

Files are lost when the following steps are made:
Start a jail foo, edit its /etc/fstab.foo,
adding a nullfs-rw  mount that is not valid (e.g. non-existent mountpoint),
jail -r jail foo
When you remove the jail,
you get an error because of the wrong entry.
The main mountpoint of the jail is now still mounted.
When  you manually umount this remaining main mountpoint,
all nullfs-rw mount files are lost.

Expected result: files would not be lost
Actual result: files are lost

Example:
# cat /etc/fstab.maygo
/usr/jail/basejail                       /usr/jail/foo              nullfs  ro
/zroot/j/foo.d/foo.homedir       /usr/jail/foo/usr/home     nullfs  rw
/zroot/j/foo.d/foo.var           /usr/jail/foo/var          nullfs  rw
/zroot/j/foo.d/foo.tmp           /usr/jail/foo/tmp          nullfs  rw
/zroot/j/foo.d/foo.etc           /usr/jail/foo/etc          nullfs  rw
/zroot/j/foo.d/foo.root          /usr/jail/foo/root         nullfs  rw

# ---------------------------
# cat /etc/jail.conf
# Common configs for all jails
  mount.fstab = "/etc/fstab.${name}";
  exec.start = "/bin/sh /etc/rc";
  exec.stop = "/bin/sh /etc/rc.shutdown";
  exec.consolelog = "/var/log/jail_${name}_console.log";
  host.hostname = "${name}";
  allow.mount;
  devfs_ruleset = "4";
  mount.devfs;
  allow.set_hostname = 0;
  path  = "/usr/jail/${name}";

foo {
  ip4.addr = 10.2.2.4 ;
  interface = vtnet0;
  allow.sysvipc = 1;
  allow.raw_sockets = 1;
  persist;
}

# -----------------------------
# jls
   JID  IP Address      Hostname                      Path
# jail -c foo
foo: created
# find /zroot/j/foo.d | wc
     100     100    3971
# jls
   JID  IP Address      Hostname                      Path
    46  10.2.2.4        foo                         /usr/jail/foo
# echo "/zroot/j/foo.d/foo.root          /usr/jail/foo/NONEXISTENTDIR         nullfs  rw" >> /etc/fstab.foo
# tail -3 /etc/fstab.foo
/zroot/j/foo.d/foo.root          /usr/jail/foo/root         nullfs  rw

/zroot/j/foo.d/foo.root          /usr/jail/foo/NONEXISTENTDIR         nullfs  rw
# jail -r foo
foo: removed
jail: foo: mount.fstab: /usr/jail/foo/NONEXISTENTDIR: No such file or directory
# mount | grep foo
zroot/j/foo.d on /zroot/j/foo.d (zfs, local, noatime, nfsv4acls)
# umount /zroot/j/foo.d
# echo $?
0
# find /zroot/j/foo.d | wc
       1       1      17
# zfs list | grep foo
zroot/j/foo.d     34.0M  8.50G  34.0M  /zroot/j/foo.d
# zfs get mountpoint zroot/j/foo.d
NAME             PROPERTY    VALUE             SOURCE
zroot/j/foo.d  mountpoint  /zroot/j/foo.d  inherited from zroot
# mount zroot/j/foo.d /zroot/j/foo.d
mount: zroot/j/foo.d: No such file or directory