The MQTT client plugin has a small bug.
When configuring the plugin for the first time "Var" is prefixed to the variables it creates, including the $MQTTClientStatus variable converting it to $VarMQTTClientStatus.
Upon re-entering the plugin config, adding another subscription name with associated variable and script, saving and then exiting the plugin, "Var" is again prefixed to all subscription variables until you eventually end up with ...
by the time you get to the seventh subscription
This is visible in the variable watcher.
I am using the latest version of ARC and MQTT Client plugins on a clean install with minimum code in place.