I tested a release upgrade from Ubuntu 16.04 to Ubuntu 18.04 today using the version of release-upgrader in -proposed and the lock screen did not appear.
bdmurray@clean-xenial-amd64:/tmp/u-r-u$ grep "release-upgrader version" /var/log/dist-upgrade/main.log
2018-07-16 14:00:39,174 INFO release-upgrader version '18.04.21' started
I tested a release upgrade from Ubuntu 16.04 to Ubuntu 18.04 today using the version of release-upgrader in -proposed and the lock screen did not appear.
bdmurray@ clean-xenial- amd64:/ tmp/u-r- u$ grep "release-upgrader version" /var/log/ dist-upgrade/ main.log
2018-07-16 14:00:39,174 INFO release-upgrader version '18.04.21' started