Get posts by name and taxonomy term

Note that if you have the post slug, you don’t need the additional taxonomy query to find that post, so it makes sense to drop the expensive taxonomy query in core in that case.

Otherwise you could try instead the post_name__in parameter:

[      
    'post_name__in' => [ '<postslug>' ],
    'tax_query' => [
        [
            'taxonomy' => '<taxonomy>',
            'field' => 'slug',
            'terms' => <slug>,
            'operator' => 'IN'
        ]
    ],      
];

that was introduced in WordPress 4.4, if you really want the taxonomy query part included as well 😉