To my knowledge, the On Update microflow should have 1 parameter with the object that is going to be updated (the Source Entity), and 1 non-persistent object which is populated based on the incoming JSON message. See also documentation snippet below that was taken from https://github.com/mendix/RestServices
The message indicating that the 2nd parameter should be 'persistable' seems like a bug to me and should read 'non-persistent' or 'transient' instead
On Update Microflow(optional)
Microflow that processes incoming changes.
Should have two parameters; one of same type as the source entity, and one which is a transient entity. The transient object will be constructed with JSON data in the incoming request. This transient object should be processed by the microflow and update the source object as desired. Use the ThrowWebserviceException
method of Community Commons to signal any exceptions to the consumer.
Could you elaborate this. How is the mapping, is it the consuming of the publishing side of the service? Are you only logging a message? Where is the service call?