I seem to have run into a sort of odd issue with WP_Query post__in I can't quite figure out. ANY help is appreciated!

Goal/Progress so far:

Overall I want to merge two arrays and use the merged array inside a WP_Query. I currently have two get_posts queries, one has 5 posts (can be any number), the second has 2000+ posts. I'm then merging these and using the merged array inside WP_Query by saying 'post__in' => $mergedArray and I want to order the posts so the 5 come first, so i'm then saying 'orderby' => 'post__in' to maintain the same order as the merged array. The issue is this only works if the $mergedArray total count is below ~1340. Otherwise the result is just blank.

I've tested a lot of different ideas so far to try to narrow down the core issue. From what i can tell at this point it has something to do with the amount of posts in my second query. Also, you will see I am using a custom taxonomy but this doesn't seem to affect the actual outcome overall. The issue persists with and without the tax_query.

My code:

         $queryOne = get_posts( 
            array( 
                'post_type' => 'post', 
                'fields' => 'ids',
                'posts_per_page' => -1,
                'tax_query' => array(
                    array(
                        'taxonomy' => 'blog_post_loc',
                        'field' => 'slug',
                        'terms' => 'blog-hub-page'
                    )
                ),
            )
        );
        $queryTwo = get_posts(
            array(
                'post_type' => 'post',
                'fields' => 'ids',
                'posts_per_page' => 1330, // for some reason if we go higher than 1330 (or use -1) it breaks
                'tax_query' => array(
                    array(
                        'taxonomy' => 'blog_post_loc',
                        'field' => 'slug',
                        'terms' => 'blog-hub-page',
                        'operator' => 'NOT IN'
                    )
                ),
            )
        );
        $mergedArray = array_merge($queryOne, $queryTwo);
        $blogLoop = new WP_Query( 
            array( 
                'post_type' => 'post',
                'post__in' => $mergedArray,
                'orderby' => 'post__in',
                'posts_per_page' => 9,
                'paged' => $paged,
                'has_archive' => true
            )
        );

List of tests i've done so far:

If I remove 'orderby' => 'post__in' then the wp_query does output all 2000+ posts, however they are not the order i'm looking for. I also checked the count to see if this was indeed the merged array and it is and i can confirm by limiting or increasing the number of posts on the first query and checking the resulting count of the $mergedArray

If I take the args from the second query and put them directly into the WP_Query args then I get back all 2000+ posts as it should, so this tells me the actual args are fine.

Even if we forget for a moment about the merging of the two arrays, if I only use the second query and I don't limit it at all, just tell it to grab all posts, then I go to use that array inside my WP_Query through 'post__in' and keep orderby as well, then it still has the same issue where its just blank. If i remove orderby then it shows all posts in the array.

If I limit the number of posts in the second query to around 1300 then everything works perfectly how I want (first query coming first, followed by second query) but i'm only getting about half of the total posts.

My assumption:

So what this all leads me to believe is either A) I'm just messing something up and im oblivious to what that is, or B) post__in and orderby has some limit on the number of posts it can handle, and/or maybe there is some param I can use to remove that limit.

Again any help at all is much appreciated. I've pretty much tested everything I can think to test and at this point if nobody else has any ideas then it may just have to stay at 1300 posts cause i'm out of options lol