This is KDE Plasma Version 5.10.5 with KDE Frameworks Version 5.38.0 on KUbuntu 17.10 with Kernel 4.8.0-46-generic. I have observed the issue for one or two years. Screen is 4k HDMI. Desktop is set to folder view (desktop folder) with custom background images per activity. Turning off (power down) the screen and turning it on again resets the background image to the stock KDE image.
Additional information: Changing the background image after that does not survive changing activities. Switching to one activity and then back shows the stock KDE background image again.
Is this still happening for you in Plasma 5.17 or the 5.18 beta?
I stopped using activities a while ago. Thus, I just added an activity for testing purposes right now and tried to reproduce the issue. I cannot observe the issue in KDE Plasma 5.16.5 with KDE Frameworks 5.62.0.
Thanks for the info!