Incorrect reply routing for ZMQ_REP/ZMQ_XREP socket

In accordance to the zmq socket description:
Each request received is fair-queued from among all clients, and each reply sent is routed to the client that issued the last request.

I use modified examples from http://www.zeromq.org/blog:multithreaded-server
http://www.zeromq.org/local--files/blog:multithreaded-server/server.cpp
http://www.zeromq.org/local--files/blog:multithreaded-server/client.cpp

Server running with one worker thread. Also two clients sends messages to server. First client without delay, second with delay 1 second between messages.

Each client sends messages with last digit of him pid. For example
Hello message: 0 7

7 - part of pid

Server sends reply with same message.

Example of second client output:
Recv msg 18: Hello message: 6 0
Send msg: OK
Recv msg 18: Hello message: 3 7
Send msg: OK
Recv msg 18: Hello message: 8 0
Send msg: OK
Recv msg 18: Hello message: 7 0
Send msg: OK
Recv msg 18: Hello message: 4 0
Send msg: OK
Recv msg 18: Hello message: 7 7
Send msg: OK
Recv msg 18: Hello message: 8 7
Send msg: OK
Recv msg 18: Hello message: 9 7

So, first client recives а replies for first client (end of pid 0)