SUMMARY

On a machine running the KDE Neon and the NVidia 515.65.01 driver in X11, with a dual-monitor setup (1 HDMI, 1 DP), the monitor connected to the DP is always disabled on resume from suspend. I don't have another system to test whether this behavior is exhibited on non-NVidia hardware. I have wiped ~/.local/share/kscreen with no change in behavior. However, the ubuntu-desktop environment does not exhibit this behavior.

On resume, when the DP monitor is disabled, it can be re-enabled manually in the System Settings Display Configuration page. However, this must be done on every resume from suspend. Prior to doing this, the desktop area will "scroll" the amount of pixels the second monitor would have provided, either horizontally or vertically depending on the displays' orientations and layout (like the old-school X11 virtual desktops that had more usable area than a single monitor supported).


STEPS TO REPRODUCE
1. Use two monitors in any orientation, with an NVidia card.
2. Suspend the machine.
3. Resume the machine.

OBSERVED RESULT

The DisplayPort monitor is disabled upon resume and must be re-enabled in System Settings.

EXPECTED RESULT

The DisplayPort monitor should remain in the same state it was in prior to resume.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 5.26.0
(available in About System)
KDE Plasma Version: 5.26.0
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.6

ADDITIONAL INFORMATION

This behavior is not experienced within the ubuntu-desktop environment, either in X11 or Wayland. This suggests the issue is not within the NVidia drivers and is not specific to the X11 protocol.