The permission difference 775 vs 755 isn't in the same place.
The chown is from the postinst, but the initial (and never changed) 775/755 is from unpack:
If I just unpack tomcat8 on Xenial and Bionic I already get:
root@x:~# ll /etc/tomcat8/Catalina/
total 3
drwxr-xr-x 3 root root 3 Feb 27 08:09 ./
drwxr-xr-x 4 root root 10 Feb 27 08:09 ../
drwxrwxr-x 2 root root 2 Jan 24 19:34 localhost/
The permission difference 775 vs 755 isn't in the same place.
The chown is from the postinst, but the initial (and never changed) 775/755 is from unpack:
If I just unpack tomcat8 on Xenial and Bionic I already get:
root@x:~# ll /etc/tomcat8/ Catalina/
total 3
drwxr-xr-x 3 root root 3 Feb 27 08:09 ./
drwxr-xr-x 4 root root 10 Feb 27 08:09 ../
drwxrwxr-x 2 root root 2 Jan 24 19:34 localhost/
root@b:~# ll /etc/tomcat8/ Catalina/
total 3
drwxr-xr-x 3 root root 3 Feb 27 08:09 ./
drwxr-xr-x 4 root root 11 Feb 27 08:09 ../
drwxr-xr-x 2 root root 2 Sep 9 19:47 localhost/