Meta query interfering with orderby relevance

I’m trying to re-order the search results using the pre_get_posts hook and set the orderby parameter to relevance. I also want to put priority posts, which is based on post meta _prioritize_s to be placed first in the results. Then all the posts after will be based on relevance as usual.

Here’s the code I’ve been messing with:

//runs on pre_get_posts
function modify_search_results_order( $query ) {
        if ( $query->is_main_query() && is_search() && ! is_admin() ) {

            $query->query_vars['order']        = 'DESC';
            $query->query_vars['orderby']      = 'relevance';
            $query->query_vars['post_status']  = 'publish';
//          $query->query_vars['meta_query']   = [
//              'relation' => 'OR',
//              array(
//                  'key'     => '_prioritize_s',
//                  'compare' => 'EXISTS'
//              ),
//              array(
//                  'key'     => '_prioritize_s',
//                  'compare' => 'NOT EXISTS'
//              )
//          ];
        }

        return $query;
    }

I originally had the orderby set to meta_value relevance and it did put the priority posts on top with the meta_query uncommented. However, it doesn’t keep the relevance sorting intact for posts afterwards. It seems like the meta query is affecting the overall order.

I have a feeling I might need something custom like a database query instead? Doesn’t seem like I’m going to get what I need by just setting the $query->query_vars. Can anyone help out? Thanks in advance!

UPDATE: Since I haven’t figured out how to alter the code I posted above, I’m trying an alternate solution. Let me know if it’s the wrong way to go about it. Leaving the above code alone, I can use the found_posts hook to alter the search results query after my pre_get_posts function has already done it’s magic. However my only issue is the second parameter that should give me the query is only giving me 10 posts when I know the results should be more than that. Why is it limiting it to 10 posts even if I set posts_per_page to -1 in my pre_get_posts function?

function modify_search_results_prioritized( $found_posts, $wp_query ) {
        if ( is_search() ) {
            //this says 10 - why?
            error_out($wp_query->query_vars['posts_per_page']);
        }

        return $found_posts;
    }

So after @Nicolai found that blurb in the WP source confirming that I can’t use multiple parameters in the orderby if I am ordering by relevance I had to go a different route. However, this route only works if I don’t care for pagination. There probably is a way to put it back, but I’m working on a different solution now and I no longer need to stay on the code I have in my question.

So I kept the function to alter the query on pre_get_posts and removed anything regarding post meta/meta queries:

function modify_search_results_order( $query ) {
        if ( $query->is_main_query() && is_search() && ! is_admin() ) {
            $get_expired_events = Event_Helpers::get_expired_event_ids();

            $query->query_vars['posts_per_page'] = - 1;
            $query->query_vars['order']          = 'DESC';
            $query->query_vars['is_search']      = true;
            $query->query_vars['post__not_in']   = $get_expired_events;
            $query->query_vars['orderby']        = 'relevance';
            $query->query_vars['post_status']    = 'publish';
        }

        return $query;
    }

Then in my search template, I call a function that alters the query after pre_get_posts to look for the priority posts, unset them from the main query, and place them back on top:

//in search.php
$wp_query->posts = Search_Modify_Query::rearrange_search_query_for_priority_relevance( $wp_query->posts );

//in PHP Class
public static function rearrange_search_query_for_priority_relevance( $query ) {
        //get prioritized ids
        $get_prioritized = self::get_priority_search_post_ids();
        $get_results_ids = [];

        if ( ! is_array( $get_prioritized ) || ! $get_prioritized ) {
            return $query;
        }

        //save all ids from current search results query
        foreach ( $query as $key => $post ) {
            $get_results_ids[ $key ] = $post->ID;
        }

        if ( ! $get_results_ids ) {
            return $query;
        }

        //check if there are priority posts in results
        if ( array_intersect( $get_prioritized, $get_results_ids ) ) {
            $priority_matches = array_intersect( $get_results_ids, $get_prioritized );
            $priority_query   = false;

            //if there are priority matches, pluck them out to put them on top
            if ( $priority_matches ) {
                foreach ( $priority_matches as $key => $priority_match ) {
                    //save these into new array first
                    $priority_query[ $key ] = $query[ $key ];
                    //then unset them from main query
                    unset( $query[ $key ] );
                }

                if ( $priority_query && is_array( $priority_query ) ) {
                    //then re-add them on top of main query
                    return array_merge( $priority_query, $query );
                } else {
                    return $query;
                }
            }
        }

        return $query;
    }

This works fine but because I needed to have ALL the results to do the comparison with my “priority post” ids vs the results ids, I had to set posts_per_page to -1. So while it works, unless I find a way to put back pagination or write something custom, the search results will give me all of the results on search.php whether it’s 5 or 500. Still, I’m placing this code here in case it helps someone else in the long run.

For my case though, I’ve decided to do two separate queries and I confirmed we don’t care about having duplicates. So I’ll query just the priority posts matching the search term above the main search results query.