I installed Arch Linux on a computer's friend with Plasma 5.8.3 and KDE applications the other day. She has two monitors attached to her computer : on the left there is a 120hz monitor at 1680x1050 resolution with the "primary" desktop (plasma panel on it), and on the right there is a 1600x900 @ 60hz monitor with no plasma panel nor widget on it. 

Everything went fine until she wanted me to change the desktop configuration, from "Desktop" to "Folder view" and add shortcuts on it. Now every time we reboot the computer, the desktop settings are messed up. Sometimes, Plasma switches the primary display and puts the left monitor configuration on the right monitor (wallpaper, plasma panel, everything goes to the right monitor), and the left monitor gets the  default configuration (default wallpaper etc.). 

Some other times, everything's fine on the left monitor, but the right monitor's configuration is reset (default wallpaper etc.) : that's what happens most often.

I can eventually get back to the good configuration if I type "killall plasmashell && plasmashell" in Krunner, but I have to do it several times (between 4 and 10 times !) before getting the good setup.

Of course I tried several times to remove every related configuration file (.config/plasma*) but it does not fix anything.

The computer runs up-to-date Arch linux with Plasma 5.8.3 and the free radeon driver on AMD Card (Radeon HD 7570 I think).

One thing I notice is that my own computer used to be affected by the same "random config" bug on Plasma 5.7 with Intel IGP and I fixed it by removing the Intel graphics driver, and use kernel modesetting isntead. From that day, no more bug ; so, maybe instead of a bug in Plasma, it is a bug in the free radeon driver ?