Hi! Thanks for a detailed report!
The mariadb.service file for systemd/systemctl is inherited as-is from upstream and it has a change in the most recent upstream release: https://salsa.debian.org/mariadb-team/mariadb-server/-/blob/debian/latest/support-files/mariadb.service.in Off the top of my head I don't know how systemd is designed to handle service file updates, and if this warning is supposed to trigger every single time any service file updates, and if packages are supposed to do anything more than just trigger service reloads (like done via https://salsa.debian.org/mariadb-team/mariadb-server/-/blob/debian/latest/debian/mariadb-server.triggers) or if maintainer scripts are supposed to run systemctl reloads on their own. What comes to the 'mysql.user table already exists!' warning, there are at least no changes in the maintainer script https://salsa.debian.org/mariadb-team/mariadb-server/-/blob/debian/latest/debian/mariadb-server.postinst in latest 1:11.8.2-1, nor for a long time before it either. The `--rpm --cross-bootstrap` has existed for 10+ years since https://salsa.debian.org/mariadb-team/mariadb-server/-/commit/8823defebeb9bd5c75e4398541a8cf295d5014db and are correct, albeit confusingly named upstream parameters to tell the script it is being called from a Linux packaging script. The most recent functional change to this section is https://salsa.debian.org/mariadb-team/mariadb-server/-/commit/cde8b8613e08ecb8d5f4a5de09d34418576d3040 by Sunil, but I don't see how it would cause the warnings you are seeing. Thus, I think that the mariadb-install-db section has probably emitted that warning all the time. But you are correct to point out that it is bad design to have warnings emitted to users about something users should not worry about. The `mariadb-install-db` should run only if the data directory is empty and no database exists, and otherwise `mariadb-upgrade` should run to upgrade the existing database.
