IPv6 connect/disconnect issue with mqtt in or out nodes

If you have only one subscription and you make a change that makes the MQTT node be redeployed (i.e if you modify it or use full deploy mode) you will be hitting a known bug

See here: MQTT not reconnecting if only only one mqtt node using it

And here: MQTT not reconnecting on edit if only only one mqtt node using the config node · Issue #3927 · node-red/node-red · GitHub

Fixed in next release (v3.1.0)