The root of the issue is this section on the emonSD build guide
wget http://repo.mosquitto.org/debian/mosquitto-repo.gpg.key
sudo apt-key add mosquitto-repo.gpg.key
cd /etc/apt/sources.list.d/
sudo wget http://repo.mosquitto.org/debian/mosquitto-jessie.list
sudo apt-get update
sudo apt-get install mosquitto mosquitto-clients libmosquitto-dev -y
[Another part of build guide apparently not used for the oct2018 image]
To fix this the mosquitto repo needs removing from the source list so that the Debian Stretch stable version is used from the Debian repos, that’s the version you are currently installing.
However, I’ve not had a chance to look into how to get a clean roll-back.
I do not believe that the Debian stable version can be aware of what the latest mosquitto repo version might have installed so it cannot be relied upon to “fix” (ie rollback). Therefore if it were me I would uninstall/purge before editing the sources list and then reinstall from the debian repo. However, that may require some special steps taken to preserve the config file since the build guide edits the package maintained conf rather than using a drop in.