Comment 29 for bug 1750465

i have tested the proposed package and it works (on ubuntu-MATE 17.10 clean install)

on a clean install of ubuntu-MATE 17.10 (iso from october 2017) , then the usual standard sudo apt update, sudo apt upgrade... then checking the setting in "pre-released updates" in developers options in software & updates... then running sudo apt update, sudo apt upgrade to get the proposed updates,

then running the sudo do-release-upgrade -d the upgrade to bionic beaver 18.04 works...
the upgrade to bionic beaver 18.04 (ubuntu-MATE) will not work if you do not apply the updates in the to the bionic-proposed..

in short, if you are Upgrading ubuntu-MATE from 17.10 to 18.04 you need the bionic-proposed updates... otherwise the update will not work. IMO it is fixed with the bionic-proposed.