I experienced the same with 5.1.1 installed from scratch. No disconnections but everything seemed slower and got a lot of log messages “No ACK from MQTT server in 10”. Went back to 5.1 and working fine since.
@nickrout is there anything we can do to help track this problem? I have updated to latest home assistant version 2021.04.3 and thought about upgrading mosquitto add-on to 5.1.1. As soon as I did the “No ACK from MQTT server” messages came back.
I noticed MQTT is now on 6.0.1.I figure I would update from 5.1 and see if there would be no issue. While HA seems to be working as it should. I noticed in t he supervisor log of this warning. Anyone knows how to fix it?