It is private function of the BugData class, which means the bug's id is assigned to the attribute "id" of the object. However, in this function the code goes like the following:
$query = "SELECT COUNT(*) FROM $t_bugnote_table WHERE bug_id =" . db_param() . " $t_restriction";
$result = db_query_bound( $query, Array( $this->bug_id ) );

The "bug_id" attribute doesn't exist (since it's "id") and thus, the query searches for "bug_id=0" always, returning 0 bugnotes found. It's not critical but the API is incorrect because of it.