You are pointing to the nightly builds so you are on a higher version now than Sam’s release builds.
returns: 18.8-23
Is the nightly 18.2RC1 version
The latest “official” test build from Sam is 18.2.0-4 (As you are 18.8.x you will not automatically get these)
To go back to the official releases comment out the gmc-19 line in the sources.list and run a
sudo apt-get update
You will then need to force a downgrade
sudo apt-get install vero3-mediacenter-osmc=18.2.0-4
When the stable release is released you can then upgrade:
sudo apt-get update && sudo apt-get dist-upgrade
Of course you will need to be on “stretch-devel main” branch otherwise you will not be able to install the test release. (Otherwise if you stay on “stretch main” then you will need to downgrade to the stable when its released.