paho client do not send puback to mqtt broker when cleansession=false & qos =1.

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- [x] Bug exists Release Version 1.2.5 ( Master Branch)
- [ ] Bug exists in MQTTv3 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)
- [ ] Bug exists in MQTTv5 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)

Here is the situation:
I used the paho mqttasyncclient, and connected the mqtt broker with option **"cleansession false & qos = 1"** twice, once was for pubclient, and another was for subclient. Certainly, I subcribed the correct topic with the subclient. Then I published some messages to the topic which I subcribed by the pubclient, and everything was OK.
After that, I disconnected the subclient, meanwhile I published some messages to the correct topic. Then I reconnected the subclient, then I got into the trouble. **I did receive the message I had published, but my paho mqttasyncclient did not send the puback to the mqttbroker!** And then the mqtt broker republish the same messages again and again, and I could not unsubcribe the topic then.
Does anyone have the idea what have happend to me?
Thanks a lot.