In following definition of _cpp_save_parameter of v3.4.6 (It also can be found in v4.3.0)

1238 bool
1239 _cpp_save_parameter (cpp_reader *pfile, cpp_macro *macro, cpp_hashnode *node)
1240 {
       ...
1255   node->flags |= NODE_MACRO_ARG;
1256   len = macro->paramc * sizeof (union _cpp_hashnode_value);
1257   if (len > pfile->macro_buffer_len)
1258    {
1259       pfile->macro_buffer = xrealloc (pfile->macro_buffer, len);
1260       pfile->macro_buffer_len = len;
1261    }
1262   ((union _cpp_hashnode_value *) pfile->macro_buffer)[macro->paramc - 1]
    = node->value;
1263  
1264   node->value.arg_index  = macro->paramc;
1265   return false;
1266 }

Above line 1262 saves the value of cpp_hashnode by copying the struct, per my understanding it is because the hashnode will be used by other identifiers of same name in other following definitions, by saving the value, we can restore the node when needed. However, operation done at line 1264 can not be included by the copy, and in macro_buffer, arg_index is unset. Though currently, I can't find the place using this field, I also suggest to swap line 1264 and 1262.