My system is a desktop tower with a fairly recent Intel motherboard with built-in Intel graphics. It does not have any custom Xorg configuration files - I installed 12.2 fresh on it in November and everything worked right out of the box. I upgraded to 12.3 yesterday, and now have a situation where the mouse will stop working (the cursor is frozen in its last location, and the buttons are unresponsive) after a period of inactivity, which seems to be about 30 minutes. It will not return if I hit the double ctrl-alt-bksp key sequence to restart KDE and login again. The only solution at this point is ctrl-alt-del and use the keyboard to select restarting the system.
This is not related to power management or the screen saved, as I turned off both features just to make sure. It only seems to happen if there are no open applications on the current KDE pane - that is it has not happened if I have Firefox open on a webpage with an animated icon. My mouse is a plain USB model, I think Microsoft optical.
I have some desktop effects enabled and did not try disabling that yet.