get_option() not returning expected value from plugin

It looks like your serialized settings are corrupted. I created the option mfwp_settings with the value array( 'test' => 'response' ). When I look at this in the database, the value is stored as a serialized array and looks like this:

a:1:{s:4:"test";s:8:"response";}

Here is a simple debugging function that successfully returns the expected value response for test:

add_action( 'init', 'wpse_option_test' );
function wpse_option_test() {

    // Example data. Serialized, it will look like this: a:1:{s:4:"test";s:8:"response";}
    $settings_value = array( 'test' => 'response' );

    add_option( 'mfwp_settings' );
    update_option( 'mfwp_settings', $settings_value );
    $mfwp_settings = get_option( 'mfwp_settings' )['test'];

    // Output the value for debugging. Result: string(8) "response"
    exit ( var_dump( $mfwp_settings ) );
}