I had the same problem on my large project that I currently work on. It’s case when you have collision between taxonomy and post-type slugs. Problem is that you can filter by taxonomy with adding ?taxonomy_name=term_slug and also you can see the custom post by adding ?post_type=custom_post_slug. So, that’s the same syntax, and WP doesn’t know do you want “term archive” or “single custom post”.

The only, and the right way (in my opinion) to solve this is to change either taxonomy-slug or post_type-slug (probably, it’s easier to fix and handle side-effects of renaming the taxonomy, I guess).

2nd solution

Also, I found one EXACT solution, you can find the details here. In the other words, you can set for your register_taxonomy() argument $args['query_var'] = false with which you disable query parameter speakers to act as taxonomy slug. Therefore, if you want to filter by taxonomy terms directly in URL / $_GET var, you will have to do some other magic. But, at least, you will remove collision between post_type and taxonomy names.

Optionally, you will have to flush_rewrite_rules after that in whatever way you want:

1. Add flush_rewrite_rules(); in functions.php, go to the site, refresh it several times, remove that line and save functions.php again.
2. Go to WordPress admin -> Options -> Permalink and click save.

I usually do both of it. Just in case.

3rd solution

Hook custom function to pre_get_posts(). With this method, you have to be careful in which case you will edit the query args. Put this in the functions.php.

function preQueryAction( $query ) {

    // classic approach to avoid messing queries in Admin and custom queries.
    if(is_admin() or !$query->is_main_query()) return; 
    
    // selectively filter only if you are on CPT arhive
    if(is_post_type_archive()){
        $query->set( 'speakers', false );
    }
    
    return;
    
}


add_action( 'pre_get_posts', 'preQueryAction', 1 );