Did you passed the following boot parameter while installing 18.04 from proposed? "apt-setup/proposed=true"
https://bugs.launchpad.net/ubuntu/+source/debian-installer/+bug/1773973/comments/10
Did you passed the following boot parameter while installing 18.04 from proposed? proposed= true"
"apt-setup/
https:/ /bugs.launchpad .net/ubuntu/ +source/ debian- installer/ +bug/1773973/ comments/ 10