So in summary everything else goes OK and upgrade passes, but there is one 'mariadb-server' metapackage removed which should have been kept:
Removing mariadb-server (1:10.3.27-0+deb10u1) ... <------------- Removing mariadb-server-10.3 (1:10.3.27-0+deb10u1) ... Removing mariadb-client-10.3 (1:10.3.27-0+deb10u1) ... Removing mariadb-client-core-10.3 (1:10.3.27-0+deb10u1) ... Removing mariadb-server-core-10.3 (1:10.3.27-0+deb10u1) ...
