Hi,
I had the same trouble some years ago
https://forum.kde.org/viewtopic.php?f=289&t=129691

I have 2 monitors with different resolution. It seems that on boot my widgets are arranged by the second (lower) screen resolution ,resulting rearranging the widgets from my first (primary) screen on every boot .

I am on xenial based KDE neon User Edition 5.12
plasmashell 5.12.4
Qt: 5.10.0
KDE Frameworks: 5.45.0
kf5-config: 1.0

I switched from KDE_Stable to User edition and everything is fine