Reopened the issue as I was able to reproduce it related to another issue. There is a problem in the transformation logic inside DefaultMuleMessage. There was a previous change that avoided to set transformer's output datatype as the datatype in the message when the payload was not changed during the transformation. The issue here is that the payload is a String and that value is supported on the transformer, so same value is returned without modification.
Here is the configuration snippet to reproduce it: