WP_Query not using relation key as expected and not producing any results

From the developer docs:

relation (string) – The logical relationship between each inner
meta_query array when there is more than one. Possible values are
‘AND’, ‘OR’. Do not use with a single inner meta_query array.

(Emphasis mine).

It’s a bit hacky, but you could run a separate query against the meta field, then merge the results into the default search query before The Loop thus:

$wp_query->posts = array_unique ( array_merge( $wp_query->posts, $meta_query->posts ) SORT_REGULAR );
$num = count( $wp_query->posts );
// these two should be the same in this case because posts_per_page is -1
$wp_query->post_count = $num;
$wp_query->found_posts = $num;