It’s because on this line: “wp_register_style(‘login_nocaptcha_css’, plugin_dir_url( __FILE__ ) . ‘css/style.css’, array( ‘login’ ), filemtime( __FILE__ ));” in login-nocaptcha.php.
That “array( ‘login’ )” includes example.com/wp-admin/css/login.min.css on any page that this plugin gets called/works on. It’s a dependency declaration which forces that the other CSS file gets included. You can instead pass array() instead of array(‘login’) which will remove the extra css file.
So: “wp_register_style(‘login_nocaptcha_css’, plugin_dir_url( __FILE__ ) . ‘css/style.css’, array(), filemtime( __FILE__ ));”
This plugin had reverted another CSS change people were claiming to have an issue with, but left this line. It’s inadvertently including the CSS that normally only gets included on this page: example.com/wp-login.php.
Also unrelated but this latest version on this plugin also includes an extra copy of the plugin in a /trunk/ folder which should also be removed.