SUMMARY
I use 2 40" screens with nvidia 1080 TI. One is connected via Display Port, second one via HDMI (I just didn't have another display port cable at time).

The issue only occurs when I start KDE session with both monitors connected at time of startup.

STEPS TO REPRODUCE
1. Connect one monitor via DP 
2. Connect second monitor via HDMI
3. Boot computer and start KDE session

OBSERVED RESULT
background on main monitor (via DP) is black, no icons of desktop are visible. But main panel with start menu appears and it works as expected. On second monitor the background image is normally visible.

EXPECTED RESULT
Desktop background and icons appears on main screen.

SOFTWARE VERSIONS
(available in About System)
KDE Plasma Meta Version: 5.12.5
KDE Plasma Version:  5.46.0
KDE Frameworks Version: Not sure which packages it is under Gentoo
Qt Version: 5.9.6

ADDITIONAL INFORMATION
This is non-blocking issue.

Quick fix I do:
When I disconnect second (via HDMI) screen and boot, all is good and when at that moment I connect second screen, the desktop is resized corectly and all works as expected.