I've installed KDE Plasma on Arch Linux. It is a fresh install. Here's my system info:
SYSTEM INFO
Plasma Version: 5.11.5
KDE Frameworks Version: 5.42.0
Qt Version: 5.10.0
Kernel version: 4.14.14-1-Arch
Processor: i5 6200U
RAM: 8GB
Display Manager: SDDM
The Problem: 
My desktop icon layout gets changed on every boot. The problem only occurs when I restart(or shutdown+start) my system, I mean restarting plasma-shell or logging in and out does not cause any problem.
On further research, I found that the file .config/plasma-org.kde.plasma.desktop-appletsrc gets modified, **only the positions= line is changed.**
Currently, I use this script on startup:
cp .config/plasma-org.kde.plasma.desktop-appletsrc{.bak,}
kquitapp5 plasmashell && kstart5 plasmashell