I’d like to try and help you @garybean. The constant ranting takes it’s toll though. I’m just another user like you.
Regular gasoline powers a lot of cars…
constant ranting or squeaky wheel ?
I can live with a boring rolling release distro…
(how is Rhino Linux coming along?)
is multiple year old hardware a way to avoid bugs?
time to take the TI-99/4A out of storage…
Is the UEFI Firmware (Bios) up to date? Are there any newer versions?
@garybean
Is the monitor hooked up with Hdmi or Display Port? Have you tried it with Freesync and or HDR off. Have you tried the system without the external monitor hooked up?
I haven’t run into this myself. There are other annoying non-critical bugs though. Some have suggested to create a new user. If you can login its probably a config file in your home directory. If you can chroot or tty and run journalctl -b -p err you might be able to find something interesting. ALWAYS use timeshift before updating. I’m surprised its not on the live cd yet considering rolling release.
HDMI.
Everything worked correctly before the plasma 6.2 update. As the identical issue is present on two different systems, EndeavourOS and CachyOS it appears the common component is my monitor. My computer is fairly recent and was updated running Windows in August.
With my description of the issue – no KDE developer seems to be able to respond to my issue. I suppose I could change my desktop environment. Just seems to me that the KDE developers would have knowledge of what possibly is causing this issue for me and an apparent minority of KDE users on multiple OS’s.
My computer running a Manjaro STABLE partition runs perfectly well on KDE Plasma
Have you tried using display port? Seems odd Manjaro works? Maybe some package is different? 
Manjaro STABLE does not include the plasma 6.2 update
May I suggest you to switch your login manager just to test, for example installing lxdm or gdm
you can install with
sudo pacman -S gdm
then enable the service with
sudo systemctl enable -f gdm
restart the system.
If it works (or not) try to update your system, maybe there will be a fix to the problematic package.
If you want to go back to the original plasma display manager run
sudo systemctl enable -f sddm
installed xfce and using that desktop environment
Plasma is effed up