When setting up an interface with:
iface IFACE inet6 auto
        dhcp 1
	[...]
then, the dhclient -6 is correctly started (on ifup) but
not stopped (on ifdown).

Looking at the sources, inet6.defn, we can see:
method auto
  [...]
  up
    [...]
    /sbin/dhclient -6 -v -P -pf /run/dhclient6.%iface%.pid -lf /var/lib/dhcp/dhclient6.%iface%.leases -I -df /var/lib/dhcp/dhclient.%iface%.leases %iface% \
        if (var_true("dhcp", ifd) && execable("/sbin/dhclient") && var_true("request_prefix", ifd))
    /sbin/dhclient -6 -1 -v -S -pf /run/dhclient6.%iface%.pid -lf /var/lib/dhcp/dhclient6.%iface%.leases -I -df /var/lib/dhcp/dhclient.%iface%.leases %iface% \
        elsif (var_true("dhcp", ifd) && execable("/sbin/dhclient"))
    echo 'No DHCPv6 client software found!' >&2; false \
        elsif (var_true("dhcp", ifd))

  down
    [nothing to stop dhclient...]

Should not we have something like (taken from the dhcp method):
  down
    /sbin/dhclient -6 -v -r -pf /run/dhclient6.%iface%.pid -lf /var/lib/dhcp/dhclient6.%iface%.leases -I -df /var/lib/dhcp/dhclient.%iface%.leases %iface% \
        if (var_true("dhcp", ifd) && execable("/sbin/dhclient"))
    echo 'No DHCPv6 client software found!' >&2; false \
        elsif (var_true("dhcp", ifd))

(not sure if we should test for "var_true("dhcp", ifd)" in the down
part. Perhaps it is better to always stop dhclient if any?

  Regards,
   Vincent
--- up and down scripts installed:
/etc/network/if-down.d:
total 16
-rwxr-xr-x 1 root root 1015 Apr 13  2015 avahi-autoipd
-rwxr-xr-x 1 root root  372 Nov 23  2014 openvpn
-rwxr-xr-x 1 root root  800 Jan  9  2017 postfix
-rwxr-xr-x 1 root root  256 Mar 25  2013 resolvconf
lrwxrwxrwx 1 root root   32 Oct 19 10:35 wpasupplicant -> ../../wpa_supplicant/ifupdown.sh

/etc/network/if-post-down.d:
total 8
lrwxrwxrwx 1 root root   42 Oct 11 18:28 openvswitch -> /usr/share/openvswitch/scripts/ifupdown.sh
-rwxr-xr-x 1 root root  907 Feb  4  2012 vde2
-rwxr-xr-x 1 root root 1409 Mar 24  2016 wireless-tools
lrwxrwxrwx 1 root root   32 Oct 19 10:35 wpasupplicant -> ../../wpa_supplicant/ifupdown.sh

/etc/network/if-pre-up.d:
total 16
-rwxr-xr-x 1 root root  344 Aug 21  2011 ethtool
lrwxrwxrwx 1 root root   42 Oct 11 18:28 openvswitch -> /usr/share/openvswitch/scripts/ifupdown.sh
-rwxr-xr-x 1 root root 1793 Feb  4  2012 vde2
-rwxr-xr-x 1 root root 4191 Sep 15  2018 wireless-tools
lrwxrwxrwx 1 root root   32 Oct 19 10:35 wpasupplicant -> ../../wpa_supplicant/ifupdown.sh

/etc/network/if-up.d:
total 28
-rwxr-xr-x 1 root root  817 Mar 25  2013 000resolvconf
-rwxr-xr-x 1 root root  923 Apr 13  2015 avahi-autoipd
-rwxr-xr-x 1 root root 1685 Nov 23  2014 ethtool
-rwxr-xr-x 1 root root 4938 Aug 14  2020 mountnfs
-rwxr-xr-x 1 root root  385 Jul  7  2015 openvpn
-rwxr-xr-x 1 root root 1117 Jan  9  2017 postfix
lrwxrwxrwx 1 root root   32 Oct 19 10:35 wpasupplicant -> ../../wpa_supplicant/ifupdown.sh

We believe that the bug you reported is fixed in the latest version of
ifupdown, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1000180@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Santiago Ruano Rincón <santiago@debian.org> (supplier of updated ifupdown package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Mon, 16 Jan 2023 14:57:59 +0100
Source: ifupdown
Architecture: source
Version: 0.8.40
Distribution: unstable
Urgency: medium
Maintainer: Josué Ortega <josue@debian.org>
Changed-By: Santiago Ruano Rincón <santiago@debian.org>
Closes: 998088 1000180 1022843
Changes:
 ifupdown (0.8.40) unstable; urgency=medium
 .
   [ Colin Watson ]
   * Clarify formatting of post-up and pre-down aliases in interfaces(5).
 .
   [ Vincent Bernat ]
   * Kill dhclient on down for inet6 auto method. (Closes: #1000180)
 .
   [ Santiago Ruano Rincón ]
   * networking.service: Wants ifupdown-pre.service instead of Requiring it.
     Patch by Ron <ron@debian.org>
     (Closes: #998088)
   * networking.service: Add ExecStart=/sbin/ifup -a --allow=hotplug. Patch by
     "Oleg A. Arkhangelsky" <sysoleg@yandex.ru>
     (Closes: #1022843)
   * Update d/copyright and fix Source URL
 .
   [ Marek Vasut ]
   * Add short -f option for --force long option
 .
   [ Simon Arlott ]
   * Support IPv6 tunnels (ip6ip6 and ip6gre)
   * Support encaplimit on IPv6 tunnels
   * Document and test support for ipip6 tunnels too (IPv4 in IPv6)
 .
   [ Guillem Jover ]
   * Rework execable() function to accept a filename instead of a pathname
   * Do not use absolute pathnames when executing commands
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