term_exists returns null, but wp_insert_term can’t insert because it exists

I have a problem with the functions term_exists and wp_insert_term. Say I do something like this:

       $attributeTerm = "X & Y";
        $attribute = "pa_color";

        $term = term_exists($attributeTerm, $attribute);
        WP_CLI::log(var_export($term, 1));

        if (!$term) {
            $term = wp_insert_term($attributeTerm, $attribute);

            if (is_wp_error($term)) {
                WP_CLI::log(print_r($term, 1));
            }
        }

If the slug of the term is kind of non-standard, like x-y_895604_895604, then term_exists will return NULL. So the if-block is entered. But then wp_insert_term returns a WP_Error object with the error that the term exists with the same name in that taxonomy.

Why does this happen? And is it future proof if I use the WP_Error object to get the term_id of the existing term?

I’d rather the term_exists function work correctly, though.