2 separate postfix instances and confusion with IP's in received field of mail message
Solution 1:
You didn't specify how the mails are created (before sending). As they don't have revealing Received headers I assume that they are created directly on the server. And there is the problem. They are not "created" by sending them via SMTP to the server from a client. And so the configuration in master.cf is ignored completely.
So if you "drop" the mail to a Postfix instance via sendmail or maildrop (or similar) you have to specify to which instance the mail should be "queued" for outbound delivery. Otherwise the first one is assumed.