dynamic update for string vars does not work

i noticed that when i try to do a dynamic update of the scheme in congestion control it doesn't appear to work:
registerd callback: CongestionControlDefaultSchemeChanged

this function gets called back correctly when the var:
CONFIG proxy.config.http.congestion_control.default.congestion_scheme STRING per_host

is updated and 'traffic_line -x' is called..
however the var returned via r->data (record data) or DEFAULT_congestion_scheme_str has not been updated unlike for its integer config counterparts -
Th r->data has been overwritten with rubbish - in fact tracing it back i see

P_RecCore.i calls
for (cur_callback = r->config_meta.update_cb_list; cur_callback; cur_callback = cur_callback->next) {
(*(cur_callback->update_cb)) (r->name, r->data_type, r->data, cur_callback->update_cookie);

this then calls before the callback function (r->data is correct at this point) a function called 'link_string_alloc' which has been registered via this function for all strings in general:
CC_EstablishStaticConfigStringAlloc(DEFAULT_congestion_scheme_str, "proxy.config.http.congestion_control.default.congestion_scheme");

The link_string_alloc overwrites the string with the passed in cookie var which has rubbish - this function looks completely wrong -

i have commented it out updated string in r->data doesn't get overwritten and so is passed through ok -

***could anyone explain what link_string alloc is actually needed for and does removing this lead to a memory leak?