Fix duplicate _pronamic_payment_subscription_id meta due to unprefi…

…xed meta key.

src/Payments/PaymentsDataStoreCPT.php

@@ -859,7 +859,7 @@ private function update_post_meta( $payment ) {
 		// Subscriptions.
 		$meta_key = $this->get_meta_key( 'subscription_id' );
 
-		$subscriptions_ids = \get_post_meta( $id, 'subscription_id' );
+		$subscriptions_ids = \get_post_meta( $id, $meta_key );
 
 		foreach ( $payment->get_subscriptions() as $subscription ) {
 			$subscription_id = $subscription->get_id();