SUMMARY
After returning from suspend (sleep), windows show no title bar and are unresponsive. The applications themselves seem to be running, and it is possible to interact with the task bar and K menu. Newly launched applications also show the same problem. This started happening after updating the official Kubuntu KDE packages on 21 May 2020.

Restarting the machine cures the problem, but this rather defeats the purpose of suspend mode!

STEPS TO REPRODUCE
1. Open some applications (e.g. Firefox, Dolphin)
2. K menu > Leave > Sleep
3. Wake machine up again

OBSERVED RESULT
Application windows are displayed at original size but moved to top left of screen. No title bar is displayed on the window, so they cannot be moved or closed. Windows appear to be unresponsive. Clicking on a window does not activate it or divert focus from the desktop, so typing brings up the desktop search bar. The K menu and task bar work as expected, but newly launched applications also lack title bars and cannot be moved or interacted with, which is why I think that this is a problem with the window manager.

EXPECTED RESULT
Applications are restored to state before suspend

SOFTWARE/OS VERSIONS
Windows: -
macOS: -
Linux/KDE Plasma: Kubuntu 20.04
(available in About System)
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

ADDITIONAL INFORMATION
The full version number reported for kwin-x11 is 5.18.5-0ubuntu0.1
Kernel version: 5.4.0-31-generic
OS Type: 64 bit
Processors: 4x Intel Core i3-6100 CPU @ 3.70GHz
Memory: 15.6 GiB of RAM
Graphics: GM206 (GeForce GTX 950) OpenGL version 4.6.0 NVIDIA 440.64
Monitor: 4K HDMI