Currently, the "synchronous" attribute on an endpoint affects Mule on the Event level (not the endpoint). This causes a problem when applying a forwarding catch-all strategy that forwards to the same endpoint, but tries to set it as "asynchronous".
In other words, under certain circumstances, a customer wants to forward messages from the default inbound endpoint to the same endpoint URL, but wants to make it asynchronous instead of synchronous. Unfortunately, "synchronous" is set on the event level, and setting the "synchronous" attribute on the second endpoint has no effect.
Weiwei has developed a quick fix (to DefaultInboundRouterCollection), and successfully applied it to the customer's installation. I will attach it to this record.
I will also post a config/test to demonstrate the issue as soon as it is available.