After researching further, I reviewed file /usr/lib/systemd/system/postgresql-11.service and found the following offending line: Environment=PGDATA=/var/lib/pgsql/11/data/
Which should have been set to /postgres/data/datafiles based in the option provided to the initdb command.
I'm curious to know where you got the idea that initdb integrated with systemd. I'm under the impression it doesn't. PostgreSQL core doesn't generally deal with such platform-specific concerns (i.e., it isn't a feature enhancement we would consider).
If you go against the conventions of your packaged distribution (or do not avail yourself of its tooling) you have to do the customizations yourself.
