Why is my nested ‘OR’ meta_query not working?

Just a couple of minor formatting changes: an extra nested array that can be deleted, and the incorrect 'value' when using the '=' compare operator, and an unnecessary 'relation' parameter.

$args['meta_query'] = array(
    // array( <- an extra array you don't need
        'relation' => 'OR',
        array(
            'relation' => 'AND',
            array(
                'key' => 'dummy',
                'value' => [false, null], // <- When using the '=' compare operator, the value cannot be an array, use an 'IN' compare value instead to see if the value of 'dummy' is 'IN' this array
                'compare' => '=',
            ),
            array(
                'key' => 'merk',
                'value' => $machine['machine_merk'][0],
                'compare' => 'LIKE',
            ),
            array(
                'key' => 'merk_type',
                'value' => $machine['machine_type'][0],
                'compare' => '=',
            ),
        ),
        // array( <-------------------|
            // 'relation' => 'AND', <-| Unnecessary as there is only one array being compared here
            array(
                'key' => 'dummy',
                'value' => true,
                'compare' => '=',
            ),
        //)
    //)
);

So it should look like this:

$args['meta_query'] = array(
    'relation' => 'OR',
    array(
        'relation' => 'AND',
        array(
            'key' => 'dummy',
            'value' => [false, null],
            'compare' => 'IN',
        ),
        array(
            'key' => 'merk',
            'value' => $machine['machine_merk'][0],
            'compare' => 'LIKE',
        ),
        array(
            'key' => 'merk_type',
            'value' => $machine['machine_type'][0],
            'compare' => '=',
        ),
    ),
    array(
        'key' => 'dummy',
        'value' => true,
        'compare' => '=',
    ),
);

Source: WP_Meta_Query