I solved the issue the following way:
create new file in "/etc/apt/preferences.d/" named "mariadb":
Package: *
Pin: origin ftp.osuosl.org
Pin-Priority: 1000
details: https://kb.askmonty.org/en/installing-mariadb-deb-files/#pinning-the-mariadb-repository
This does Pin the MariaDB Repository and gives it a very high priority above 999 priority means it would even downgrade packages to install MariaDB.
Reason:
Oracle is issuing security update for MySQL and Ubuntu is distributing very up-to-date MySQL packages with higher version number than MariaDB. And MariaDB (it's a fork of MySQL) provides several binaries under the same name as MySQL. So pinning the MariaDB repository fixed the problem.
Thank goes to Jbboin of the official IRC channel who pointed me in the right direction!