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()