Once again I fail to follow the repeated almost religious secret of $LIST() encoding
that is celebrated by Support and Engineering over decades.
Especially when the use of the knowledge is labeled "illegal".
That's just disappointing.

The problem of a possible unexpected change rather indicates incomplete Release Notes to me. 

Do you expect to save #1) the input JSON object or
#2) a dump of the converted result?
For both:

set stream=##class( %Stream.GlobalCharacter).%New()

#1)  with myjson as your JSON input

do stream.Write(myjson)
do stream.%Save()  

#2) dump the generated Object  obj  using my package Full-OBJ-Dump described here

do ##class(Z.obj).dumpToStream(.stream,obj,1)
do stream.%Save()  

Status= 1 justteslls you tht the HTTP connection was processed OK.
Next you need to take a look into your response object.

set res=Httprequest.HttpResponse
in terminal then ZW res to see in the Status is 200 OK
otherwise what you see I  res.Data might be just an error page.
Which is also OK for HTTP but not for your content.

• property ReasonPhrase as %String;

This is the human readable reason that goes with the StatusCode.

• property StatusCode as %Integer;

The HTTP status code. This is useful to determine if the request was successful. Look in the rfc for HTTP to see which codes are supported and what they mean. A human readable form of this code is stored as the ReasonPhrase

• property StatusLine as %String;

The HTTP status line. This is the first line of the response and signals if the request was successful or if there was a problem.