json-read-from-string doesn't parse strings correctly if the the \u
syntax is used to write UTF-16 surrogates:

 (equal (json-read-from-string "\"\\uD834\\uDD1E\"") "\"\U0001D11E\"")
 => nil

The correct result t.  To quote RFC 7159[*]:

   To escape an extended character that is not in the Basic Multilingual
   Plane, the character is represented as a 12-character sequence,
   encoding the UTF-16 surrogate pair.  So, for example, a string
   containing only the G clef character (U+1D11E) may be represented as
   "\uD834\uDD1E".

[*] https://tools.ietf.org/html/rfc7159#section-7