`echo <something> | pfctl -mf -` overriding instead of modifying

[Niklaas Baudet von Gersdorff]

Note: crossposting in freebsd-questions and freebsd-pf

On a 10.3-RELEASE system, in my `/etc/pf.conf` I have the following lines:

	ext_if="vtnet0"
	...
    rdr-anchor "jails/*" on $ext_if inet to $ext_if

In my `/etc/jail.conf` I have the following lines for some jail:

    exec.poststart += "echo 'rdr pass on vtnet0 inet  proto { udp tcp } to vtnet0 port domain ->   $private_ip4' | pfctl -a 'jails/$name' -f -";
    exec.poststart += "echo 'rdr pass on vtnet0 inet6 proto { udp tcp } to vtnet0 port domain ->   $private_ip6' | pfctl -a 'jails/$name' -mf -";

Nonetheless, if I start the jail, only the inet6 rules will stay in the
appropriate anchor. The inet rules will be overridden.

Initially, I only used the `-f -` flags for pfctl (instead of `-mf -`) and
realised that making changes to the anchor overrides existing rules. So
I read pfctl(8) where it says

     -m      Merge in explicitly given options without resetting those
     which are omitted.  Allows single options to be modified without
     disturbing the others:

        # echo "set loginterface fxp0" | pfctl -mf -

So I thought that adding `-m` to the rule in the second `exec.poststart`
will include (instead of replace) the rules into the anchor. But this is
not the case. What am I doing wrong? Do I misunderstand `-m`?
    
    Niklaas