My PC has the following configuration:

Debian 11 bullseye
kernel 5.10.0-10-amd64
KDE Plasma 5.20.5
KDE Frameworks 5.78.0
Qt 5.15.2
X version 11.0 & OpenGL
Graphics Card Zotac GeForce GTX 460

I have two monitors connected to the graphics cards with DVI cables. The secondary display is positioned in Portrait mode (rotated sideways) on the left side of the main display. This configuration has been running fine for several months, but for an unknown reason it stopped working.

Now, every time I start my PC, after completion of the Plasma startup, the secondary display appears as if it was in Landscape mode on the right side of the main display (which I guess is the default position).

The odd thing is the following: if I unplug the DVI cable from the secondary display, the screen flickers and the windows opened on the secondary display move to the main one, as expected; if at that point I plug back the DVI cable in the secondary display, everything gets back as should be (secondary display in portrait mode, on the left) and the opened windows jump back to their correct position on the secondary display!

It is as if the display configuration was recorded correctly but was not read when Plasma starts, but a hardware change (umplugging/ replugging the cable) triggered the reading of the display configuration.

Any ideas to help me understand what's going on and to correct it are most welcome!