Values with angle brackets

Hi,

I'm wondering why the playground omits values between < and > in the output.
My example input:
{
  "@context": {
    "message-id": "http://www.semanticdesktop.org/ontologies/nmo#messageId"
  },
  "message-id":
"<CAERg+cg3y+bKtk9KE5UMmK15+1+U191oUgVZaT=ifhB7YqaMAQ@mail.gmail.com>"
}

The output (in Compacted form):

{
  "http://www.semanticdesktop.org/ontologies/nmo#messageId": ""
}

If I put text before / after the brackets, it becomes part of the
output. Do I need to escape the brackets? If so, why is that and where
is it documented?

-- Niklas Cathor