Created attachment 139032 [details]
Commented and edited screenshot to show the bug

SUMMARY
I have 2 monitors connected, primary on the left and secondary on the right. I regularly turn the secondary monitor off to save power.
After re-enabling it, the alignment of the monitors is 1 pixel off, so that the most right pixel row of the left monitor is displayed on the right monitor.
In other words, if I have a maximized window on the left monitor, the right pixel border of that window is displayed on the right monitor instead of the left.
The attached screenshot may explain this bug a little better

STEPS TO REPRODUCE
1. Connect two monitors, primary on the left and secondary on the right.
2. Disable the right monitor and immediately re-enable it.

OBSERVED RESULT
The monitors are aligned as described in the summary.

EXPECTED RESULT
The monitors should be aligned as before.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.21.5, Manjaro
(available in About System)
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.82.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
There is also  another bug after re-enabling a monitor: the wrong refresh rate is selected (50 Hz instead of 60 Hz). I will open another bug report for this.