From what I understand is that the post_meta key has been removed from mapping.
So instead of post_meta.field you should use meta.field.value (or raw etc.)
I had to change a lot in my scripts. So not too nice but fixable.
I just changed the slug of the order recieved page to the one that mollie gave back.
In case you want to find out: At woocommerce -> settings -> (tab)logs (logboeken), you can choose a transaction and see where the customer has been directed to.
I believe (this is speculation though), that the slug is being (partially) translated. So in my case (dutch) it became “order-ontvangen”.
I hope this helps.
I found that the mollie API sent the clients to a different URL than specified in the endpoints. My bad.
