Package: rt4-db-mysql Version: 4.4.4+dfsg-2 request-tracker4 as shipped with Ubuntu 20.04 is broken when used with the MySQL release also shipped in Ubuntu. Whilst we can't fix that directly in Debian, we an arrange for the dependencies to correctly reflect this, maybe by conflicting with mysql-server-8.0, and/orrequiring mariadb explicitly.
Ubuntu bug report: https://bugs.launchpad.net/ubuntu/+source/request-tracker4/+bug/1923264