Query set using tax_query with relation ‘OR’ modifies category object?

(Update: Please see the edits far below.)

The code:

function itsme_better_editions( $query ) {
    if ( $query->is_category() && $query->is_main_query() ) {
        $query->set( 'post_type', array( 'post' ) );
        $query->set( 'tax_query',
            array(
                'relation' => 'OR',
                array(
                    'taxonomy' => 'category',
                    'field' => 'slug',
                    'terms' => 'intl',
                    'operator' => 'IN'
                )
            )
        );
    }
    return $query;
}
add_filter( 'pre_get_posts', 'itsme_better_editions' );

I am using this function to display posts assigned to ‘International (intl)’ category under all categories i.e. in all category archives.

(In my case categories represent Editions as common with a news site, for example.)

The problem is, with the function in place, it appears as though the term object of all archives gets set to ‘International (intl)’, be it a category archive, tag archive, custom taxonomy archive, etc.

So, for example, if I visit the ‘UK (uk)’ category archive, the <?php single_cat_title(); ?> displays ‘International’ instead of ‘UK’. (NOTE: single_cat_title() is just an example, and only one of the problems caused by the change in term object.)

Why is this happening? (Despite me setting 'relation' => 'OR'.) What should I do to fix this?

PS: Surprisingly this used to work (very positive), and I don’t know in which version of WordPress this broke.

EDIT: The problem, I just realized, appears to be that $tax_query = $query->get( 'tax_query' ); is empty or not an array. I am not really sure why this is happening, and what’s wrong with my code.

Related Question: When/why does '$query->get( 'tax_query' );' return empty?