Go to: /etc/apt/sources.list and search at the end of the file for the MariaDB repo, something like this:
deb [arch=amd64,i386,ppc64el] http://mirrors.syringanetworks.net/mariadb/repo/10.3/ubuntu bionic main
Change the i386 for arm64:
deb [arch=amd64,arm64,ppc64el] http://mirrors.syringanetworks.net/mariadb/repo/10.3/ubuntu bionic main
This should fix the "warning" you are getting.
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