Proof http://imgur.com/DOea9qC Reproducible: Always
Could that be caused by Qt's scaling? Does adding QCoreApplication::setAttribute(Qt::AA_DisableHighDpiScaling); to plasma-workspace/ksmserver/logout-greeter/main.cpp help? Can't reproduce here, even with scaling, though.
I can confirm this issue on my Dell XPS 9350 running KDE neon 5.8 After I change scaling some menus look distorted, e.g. the login or shut down menu, or even the scaling menu which looked fine before. http://prntscr.com/cshz24
This is definitely caused by Qt's scaling, which btw doesn't seem to work at all although it had worked earlier at some point in time. It also blurs other texts on my system. Neon 5.8.2
Should be fixed by https://cgit.kde.org/plasma-workspace.git/commit/?id=4b2abc581c6b3e7a4c2f1f893d47fad5d3806aca