json_object_double_to_json_string() uses the %f conversion specifier to convert a double to a string:

  size = snprintf(buf, 128, "%f", jso->o.c_double);
  p = strchr(buf, ',');
  if (p) {
    *p = '.';
  } else {
    p = strchr(buf, '.');
  }

This is lossy, it does not preserve all significant digits.  Exact conversion between binary and decimal floating point is difficult, and json-c should probably use gdtoa or grisu to implement this.  That would ensure nice-looking but accurate output, but it's a lot of code.  There is a width (18?) that ensures that %s is completely accurate, but I don't have the details at hand right now.

The decimal comma rewriting is also a bit suspect, but I couldn't get it to fail in a fa_IR locale (which is supposed to have neither a decimal point nor a decimal comma).