meta_key and meta_value not working together

You could try this:

$args = array(
    'post_type' => 'cqpp_interventions',
    'posts_per_page' => '-1',
    'meta_query' => array(
        array(
            'key' => 'priority',
            'value' => '80'
        )
    )
);

The real issue with your query is because you are passing meta_key and meta_value. However, the arrays in your meta_query argument should have the keys key and value instead.

This would also work:

'key' => 'priority',
'value' => array('80')