I'm looking at admin/view_subscriptions.php, notably lines ~123 where you can see that it checks the User Display Name vs the User Login Name field of the database, but the wording on the admin page itself mentions Username which seems to be causing confusion.
Code
	$filter_array['username'] = $username;
	if ($username != "") {
		$query = "
			select USER_ID
			from {$config['TABLE_PREFIX']}USERS
			where USER_DISPLAY_NAME = ?
		";

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