Shreded system_health ExEvent not returning all node information
According to http://www.red-gate.com/community/books/accidental-dba [Troubleshooting SQL Server: A Guide for the Accidental DBA] this looks to be by design.

Changes to Extended Events in SQL Server Denali
At the time of the
final edits of this chapter, SQL Server Denali CTP3 was released, with
changes associated to how the Extended Events targets store XML data
inside of the value element of the Event XML  output. Listing 7.5
shows the use of the .value() method from XML in SQL Server, but in
Denali CTP3, a .query() method has to be used to retrieve the
deadlock graph from the Event XML output.

Additional information : Extended Events Changes in SQL Server 2012 – Event XML for XML data elements