After https://www.drupal.org/project/drupal/issues/3120124 went in making MariaDB 10.3 the minimum version, an install reports "Drupal already installed" if MariaDB version is 10.2 and settings.php is already set up with database settings.
To recreate (general):
1. Configure current Drupal 9 (after https://www.drupal.org/project/drupal/issues/3120124 went in), composer install, etc.
2. Configure the settings.php pointing to an empty MariaDB 10.2 database.
3. Try to install using web installer.
You get "Drupal already installed" instead of "Need MariaDB 10.3+"
Note that if you try to do an install *without* database settings in settings.php, it will properly detect that MariaDB 10.2 is out there and will refuse to go farther in the install.
To recreate with ddev:
git clone https://git.drupal.org/project/drupal.git testdb && cd testdb && git checkout origin/9.0.x
ddev config --project-type=drupal8 --php-version=7.3 --mariadb-version=10.2
ddev composer install
ddev start
ddev launch
You'll get the dreaded "Drupal already installed" after it redirects to /install.php