Comment 6 for bug 1820288

On install it already changes from:
drwxr-xr-x 2 root root 2 Sep 9 19:47 localhost/
(unpack)
to
drwxr-xr-x 2 root tomcat8 2 Sep 9 19:47 localhost/
(after postinst and service start)

This is confirmed by doing it without postinst:
root@b:~# apt download tomcat8
Get:1 http://archive.ubuntu.com/ubuntu bionic-updates/universe amd64 tomcat8 all 8.5.39-1ubuntu1~18.04.3 [46.4 kB]
Fetched 46.4 kB in 0s (449 kB/s)

root@b:~# sudo dpkg --unpack tomcat8_8.5.39-1ubuntu1~18.04.3_all.deb
Selecting previously unselected package tomcat8.
(Reading database ... 42649 files and directories currently installed.)
Preparing to unpack tomcat8_8.5.39-1ubuntu1~18.04.3_all.deb ...
Unpacking tomcat8 (8.5.39-1ubuntu1~18.04.3) ...
Processing triggers for systemd (237-3ubuntu10.38) ...
Processing triggers for ureadahead (0.100.0-21) ...
Processing triggers for rsyslog (8.32.0-1ubuntu4) ...

root@b:~# sudo mv /var/lib/dpkg/info/tomcat8.postinst /root/tomcat8-manual.postinst

root@b:~# sudo dpkg --configure tomcat8
Setting up tomcat8 (8.5.39-1ubuntu1~18.04.3) ...
Processing triggers for systemd (237-3ubuntu10.38) ...
Processing triggers for ureadahead (0.100.0-21) ...
Processing triggers for rsyslog (8.32.0-1ubuntu4) ...

root@b:~# ll /var/lib/tomcat8/conf/Catalina/
total 3
drwxr-xr-x 3 root root 3 Feb 27 07:47 ./
drwxr-xr-x 4 root root 11 Feb 27 07:48 ../
drwxr-xr-x 2 root root 2 Sep 9 19:47 localhost/
root@b:~#

Lets take apart the rather complex postinst which command will change the ownership