I had a problem with Node-red continually disconnecting from Mosquitto.
I don't know if it's the same as your issue but I was on Node-red 3.0.2, the problem went away with NR 3.1.
Oh, look, you are on 3.0.2!
Your node.js is out of date, your Node-red is out of date and your Linux is out of date.
Your Mosquitto might be out of date too - mine is 2.0.11 but I'm not sure if that's up to date either
You could update the OS with sudo apt update && sudo apt -y full-upgrade, then re-run the NR installation script to update node.js and Node-red.
Alternatively get a new uSD card, burn the latest RPiOS 64 bit, install Node-red, Mosquitto and any other services then transfer your flows.
