You need to mention the network stack your are using, NetX/NetX_Duo/LwIP?
Which MQTT broker are you using for test.
How long for the connection to get lost. (Is it hard or easy to reproduce?)
I am using using LwIP network stack, FreeRTOS.
MQTT broker version 1.4.1. After 5 to 10 mins it will break the connection. It is easy to reproduce.