Orderby post__in Not Working Correctly?

If you look at the source of the Ajax Load More plugin, you can see a number of filters you can use to modify the query. You’ll likely want to use the alm_modify_query_args filter like so:

$in_array = [ 115, 123, 66, 64 ];
add_filter( 'alm_modify_query_args', function( $args ) use ( $in_array ) {
  $args[ 'post__in' ] = $in_array;
  $args[ 'orderby' ]  = 'post__in';
  return $args;
} );

Had the developer not been so nice and provided this filter, you could have always used the pre_get_posts filter.