MariaDB and MySQL's code bases are diverging. And there is Percona Server for MySQL.
While the code bases are diverging, it remains manageable. Thus, as of 2023 and for the foreseeable future, we plan to continue to support all 3 equally. Meaning that any issue on should be reported, prioritized and resolved.
Tiki is (like Virtualmin) a component of WikiSuite.org and Virtualmin defaults to MariaDB.
I posted about a related issue here (Tiki database was exported from MySQL and imported to MariaDB and this produced some errors): https://matrix.to/#/!SLEgqpTNTlGYAhzNvK:gitter.im/$PblMCml-uxFXzDGQXkXHWYaMnLfgJnk5ehPp2n-8C34?via=gitter.im&via=matrix.org&via=sans-nuage.fr