Index: src/includes/topics/functions.php
--- src/includes/topics/functions.php Base (BASE)
+++ src/includes/topics/functions.php Locally Modified (Based On LOCAL)
@@ -2982,9 +2982,16 @@
 	delete_post_meta( $topic_id, '_bbp_engagement' );
 
 	// Query the DB to get voices in this topic
+	// See: https://bbpress.trac.wordpress.org/ticket/3083
 	$bbp_db  = bbp_db();
-	$sql     = "SELECT DISTINCT post_author FROM {$bbp_db->posts} WHERE ( post_parent = %d AND post_status = %s AND post_type = %s ) OR ( ID = %d AND post_type = %s )";
-	$query   = $bbp_db->prepare( $sql, $topic_id, bbp_get_public_status_id(), bbp_get_reply_post_type(), $topic_id, bbp_get_topic_post_type() );
+	$sql     = "SELECT DISTINCT( post_author ) FROM ( 
+					SELECT post_author FROM {$bbp_db->posts} 
+						WHERE ( ID = %d AND post_type = %s ) 
+					UNION 
+					SELECT post_author FROM {$bbp_db->posts}  
+						WHERE ( post_parent = %d AND post_status = %s AND post_type = %s )
+				) as u1";
+	$query   = $bbp_db->prepare( $sql, $topic_id, bbp_get_topic_post_type(), $topic_id, bbp_get_public_status_id(), bbp_get_reply_post_type() );
 	$results = $bbp_db->get_col( $query );
 
 	// Parse results into voices