Message Trace outputs 'null' as Message Content if the message are not Stream or Reader types. Please take a look at org.switchyard.runtime.util.ExchangeFormatter.formatBody() static method for more detail.

The JIRA ENTESB-5113 fixed message trace encoding issue but it introduced this bug. Basically it is missing an else block to do what it did before:
content = msg.getContent(String.class);
As a result, for any non-Stream and non-Reader type messages, the string variable "content" remains null:
85 String content = null;