load balancing problem

[EXT / GFI REBOLJ Jean-Pierre]

hi,
i've a proxy  with freeradius 1.1.6 in load balacing with two back-end
radius 1.1.6

my proxy is configured like this
realm APPLI1 {
       type            = radius
       authhost        = xx.xx.xx.xx:1820
       accthost        = xx.xx.xx.xx:1821
       ldflag          = round_robin
       secret          = mysecret
}

# Server Canna PROD Accellio GPRS de Test avec Realm
realm APPLI1 {
       type            = radius
       authhost        = yy.yy.yy.yy:1820
       accthost        = yy.yy.yy.yy:1821
       ldflag          = round_robin
       secret          = mysecret
}

I see the Authentication request and response then  Accounting start an
some hours  later accounting stop on the proxy

the problem is that I see the Authentication request and response then
Accounting start on the fisrt back-end server and the accounting  stop
on the second backend server.
is this a bug or a problem of configuration ?

thanks in advance.
Regards
JPR