I’ve added the ability for users to add a category to their activity posts using the following code:

// Add Cat to Buddypress Activity Updates 

function my_custom_activity_meta_stuff( $content, $user_id, $activity_id ) {
	bp_activity_update_meta( $activity_id, 'bpcat', $_POST['bpcat'] );
}
add_action( 'bp_activity_posted_update', 'my_custom_activity_meta_stuff', 10, 3 );

function add_bpcat_form_func(){
  global $wpdb;
  $result = $wpdb->get_results ( "SELECT * FROM wp_bp_xprofile_fields WHERE id='2'" );
    foreach ( $result as $print )   { ;?>
	<div class="bpcat <?php echo $print->id;?>">
		In Search Of: <select name="bpcat" id="bpcat">
		  <option value="" disabled selected>Optional</option>
  <?php $options = $wpdb->get_results ( "SELECT * FROM wp_bp_xprofile_fields WHERE parent_id='$print->id'" );
      foreach ( $options as $option )   { ;?>
    	
    <option class="<?php echo $option->name;?>" name ="field_<?php echo $print->id;?>_match_any[]"value="<?php echo $option->name;?>"><?php echo $option->name;?></option>
          <?php } ;?> 
		</select>
	</div>
 <?php }
} ; ?>
<?php
add_action('bp_activity_post_form_options', 'add_bpcat_form_func');

Now I want those categories to be searchable in the activity feed.

To this end I’ve created a custom template in my child theme for common/search/search-form.php

<?php
/**
 * BP Object search form
 *
 * @since 3.0.0
 * @version 3.1.0
 */
?>

<div class="<?php bp_nouveau_search_container_class(); ?> bp-search" data-bp-search="<?php bp_nouveau_search_object_data_attr() ;?>">
<form action="" method="get" class="bp-dir-search-form" id="<?php bp_nouveau_search_selector_id( 'search-form' ); ?>">
<select onchange="this.form.submit()" id="<?php bp_nouveau_search_selector_id( 'search' ); ?>" name="<?php bp_nouveau_search_selector_name(); ?>">
		  <option value="" disabled selected>Filter by Category</option>
		  <option value="">All</option>
  <?php  $result = $wpdb->get_results ( "SELECT * FROM wp_bp_xprofile_fields WHERE id='2'" );
  foreach ( $result as $print );
  $options = $wpdb->get_results ( "SELECT * FROM wp_bp_xprofile_fields WHERE parent_id='$print->id'" );
      foreach ( $options as $option )   { ;?>
    	
    <option class="<?php echo $option->name;?>" name ="field_<?php echo $print->id;?>_match_any[]"value="<?php echo $option->name;?>"><?php echo $option->name;?></option>
          <?php } ;?> 
		</select>
</form>
</div>

When this is used it results in a url with a querystring like:
https://mydomain.com/activity/?activity_search=CATEGORYSELECTED

But no activity updates are shown because the activity_search query searches activity content and not meta

I want the search to apply to my activity meta, not the content of the activity, how would I go about doing this?