HTTP URI filtering and chunking?

This is a standard mis-conception that if the control flows to the end of the event clause, the default pool will somehow be re-selected. This is not the case. Whatever the last pool selected remains in effect until otherwise changed. In order to do what you want you need to change the rule like so:

when CLIENT_ACCEPTED {
   set default_pool [LB::server pool]
}
when HTTP_REQUEST {
   ...
   } else {
      pool $default_pool
   }
}