Retrieve custom post types by custom taxonomies with WP_Query

You need to pass the field and terms to get this working the right way:

'tax_query' => array(
    'relation' => 'AND',
    array(
        'taxonomy' => 'provincia',
        'field'    => 'slug',
        'terms'    => array( 'term1', 'term2' ),
    ),
),

https://codex.wordpress.org/Class_Reference/WP_Query#Taxonomy_Parameters