I solved this issue on an upgraded OH3 installation with a method that was not mentioned above. There were two broker connections. One as a thing/bridge, and the other as a non-removable system connection under settings. The latter was making the errors, and I disabled it by setting Broker Host to blank, instead of 127.0.0.1.
Specifically: OH3 GUI => Settings => Other Services => MQTT system broker connection => clear the field Broker Host and Save.