I think it's fine to leave the debian bug reference, however for clarity I'd suggest changing 'Closes' to something like 'Debian BTS' or similar.  Debian's 6-digit bug numbers are close enough to Launchpad's that confusion is not unlikely; mentioning Deb or Debian somewhere will be enough clue.

Not sure if a vm is required for proper testing, but I manually built and installed postfix_3.5.13-1ubuntu2_amd64.deb, and ran the checks mentioned in the MP description and got similar results:

root@merges-jammy:/home/bryce/pkg/Postfix/review-mp412054# ip link set eth0 down
root@merges-jammy:/home/bryce/pkg/Postfix/review-mp412054# ip link set eth0 up

-- Journal begins at Tue 2021-11-16 18:45:49 UTC. --
Nov 18 17:26:08 merges-jammy postfix/master[484944]: reload -- version 3.5.13, configuration /etc/postfix
Nov 18 17:26:08 merges-jammy systemd[1]: Reloaded Postfix Mail Transport Agent (instance -).
Nov 18 17:26:08 merges-jammy systemd[1]: Reloading Postfix Mail Transport Agent.
Nov 18 17:26:08 merges-jammy systemd[1]: Reloaded Postfix Mail Transport Agent.
Nov 18 17:26:09 merges-jammy systemd-networkd[146]: eth0: Gained IPv6LL
Nov 18 17:26:16 merges-jammy sudo[485862]:    bryce : TTY=pts/3 ; PWD=/home/bryce/pkg/Postfix/review-mp412054 ; USER=root ; COMMAND=/bin/bash
Nov 18 17:26:16 merges-jammy sudo[485862]: pam_unix(sudo:session): session opened for user root by (uid=1000)
Nov 18 17:26:38 merges-jammy systemd[1]: systemd-hostnamed.service: Deactivated successfully.
Nov 18 17:27:12 merges-jammy sudo[485876]:    bryce : TTY=pts/1 ; PWD=/home/bryce/pkg/Dovecot/fix-lp1945763/dovecot-gu ; USER=root ; COMMAND=/bin/bash
Nov 18 17:27:12 merges-jammy sudo[485876]: pam_unix(sudo:session): session opened for user root by (uid=1000)
Nov 18 17:27:41 merges-jammy systemd-networkd[146]: eth0: Link DOWN
Nov 18 17:27:41 merges-jammy systemd-networkd[146]: eth0: Lost carrier
Nov 18 17:27:41 merges-jammy systemd-networkd[146]: eth0: DHCP lease lost
Nov 18 17:27:41 merges-jammy systemd-networkd[146]: eth0: DHCPv6 lease lost
Nov 18 17:27:41 merges-jammy dbus-daemon[164]: [system] Activating via systemd: service name='org.freedesktop.hostname1' unit='dbus-org.freedesktop.hostname1.service' requested by ':1.0' (uid=100 pid=146 comm="/lib/systemd/systemd-networkd " label="unconfined")
Nov 18 17:27:41 merges-jammy systemd[1]: Starting Hostname Service...
Nov 18 17:27:41 merges-jammy systemd[1]: Reloading Postfix Mail Transport Agent (instance -).
Nov 18 17:27:41 merges-jammy dbus-daemon[164]: [system] Successfully activated service 'org.freedesktop.hostname1'
Nov 18 17:27:41 merges-jammy systemd[1]: Started Hostname Service.
Nov 18 17:27:41 merges-jammy systemd-hostnamed[485888]: Hostname set to <merges-jammy> (static)
Nov 18 17:27:41 merges-jammy postfix/postfix-script[485908]: refreshing the Postfix mail system
Nov 18 17:27:41 merges-jammy postfix/master[484944]: reload -- version 3.5.13, configuration /etc/postfix
Nov 18 17:27:41 merges-jammy systemd[1]: Reloaded Postfix Mail Transport Agent (instance -).
Nov 18 17:27:41 merges-jammy systemd[1]: Reloading Postfix Mail Transport Agent.
Nov 18 17:27:41 merges-jammy systemd[1]: Reloaded Postfix Mail Transport Agent.
Nov 18 17:27:51 merges-jammy systemd-networkd[146]: eth0: Link UP
Nov 18 17:27:51 merges-jammy systemd-networkd[146]: eth0: Gained carrier
Nov 18 17:27:51 merges-jammy systemd-networkd[146]: eth0: DHCPv4 address 10.69.244.169/24 via 10.69.244.1
Nov 18 17:27:51 merges-jammy systemd[1]: Reloading Postfix Mail Transport Agent (instance -).
Nov 18 17:27:51 merges-jammy postfix/postfix-script[485937]: refreshing the Postfix mail system
Nov 18 17:27:51 merges-jammy postfix/master[484944]: reload -- version 3.5.13, configuration /etc/postfix
Nov 18 17:27:51 merges-jammy systemd[1]: Reloaded Postfix Mail Transport Agent (instance -).
Nov 18 17:27:51 merges-jammy systemd[1]: Reloading Postfix Mail Transport Agent.
Nov 18 17:27:51 merges-jammy systemd[1]: Reloaded Postfix Mail Transport Agent.
Nov 18 17:27:53 merges-jammy systemd-networkd[146]: eth0: Gained IPv6LL
Nov 18 17:28:21 merges-jammy systemd[1]: systemd-hostnamed.service: Deactivated successfully.

I didn't run the autopkgtests but trust your results.  The packaging and debian changes all LGTM, +1.