I don’t know the answer, but lately Plasma changed how the multimonitor system works entirely, so when you use two DEs, there might be conflicts and one will overwrite the other. If possible, I would get rid of LXQT components that detect and set monitors, use Kwin again and see if the issue still occurs.
You can also set a test user with default settings, log in and see if the issue occurs there as well. If not, the problem is with your configs on user space. If the problem is still there, it’s some upstream issue or conflict with packages. I suspect that if Plasma devs came out with stable and modern monitor managing system, they didn’t take other DEs into account, since mixing DEs is rather rare nowadays and not encouraged. Ironically, LXQT is a DE that works quite well with Kwin, but I guess the new settings may have a problem with it.