MDEV-34191 solved a stopped systemd service from being started on installation.

https://github.com/MariaDB/server/pull/3268 describes the jump in compat to 12, that was wound down to 11 due to Debian 10 and Ubuntu 20.04

test on focal / jammy where no immediately available after an upgrade from an earlier MariaDB provided package.

The cause was tracked down to debian/mariadb-server-10.5.prerm which does a deb-systemd-invoke stop mariadb.service.

Rather than accepting this an edge case to the desired functionality of MDEV-34191, a hasty push of https://github.com/MariaDB/server/commit/5a6b8103cd0da15650f429f493b1a42f276b796e was made to fix it. Missing from this fix was any validation that MDEV-34191 was actually corrected, and as such MDBF-771 created be lately.

Under the text of compat 11 (man 7 debhelper-compat-upgrade-checklist), because this mode is discourages, is the text "The dh_systemd_enable and dh_systemd_start helpers have been replaced by the new dh_installsystemd helper". dh_installsystemd is the rules, but now that its bumped down to 10 it probably has no effect at all.

With Debian 10 now EOL. maybe something can be done.

Maybe something like DH_COMPAT=12 dh_install_systemd
ref: https://lists.debian.org/debian-release/2019/04/msg01459.html