Thanks for your help so far.
I was able to try this with Mosquitto 1.5.6 and unfortunately, I face the same issue. It disconnects with the same frequency.
But this time I do not see any error message at Broker end.
Update: Setting the QoS to 1UL exhibits a continuous connection for around 20-25 minutes, as you rightly mentioned. Is there a way to improve in this further?
Setting QoS to 0 doesn't seem to work well. Currently I am facing the same issues you are. However I have not tried changing the protocol version. The Eclipse Paho Embedded C Client, which is the base for our MQTT client hasn´t been mantained as has Eclipse Mosquitto broker. My current strategy is to detect the disconnection events and completely restart communications and the connection to the broker.