On openmamba the SDDM login screen runs inside kwin_wayland by default (/etc/sddm.conf.d/10-wayland.conf, shipped by kwin). The nvidia_470 package overrides that with its own /etc/sddm.conf.d/nvidia_470.conf, which switches SDDM to X11, because the 470 driver doesn't work with Wayland. When you removed nvidia_470, that override went away too. SDDM went back to KWin on Wayland, and so it hits the same KWin bug you saw in the Plasma session.

Since you're on LXQt with X11, you can get the old behaviour back by creating /etc/sddm.conf.d/20-x11.conf with:

[General]
DisplayServer=x11

Then reboot, or run sudo systemctl restart sddm (this ends the current session).

Upstream has tracked down the cause: KWin 6.8 has a problem with legacy modesetting, which nouveau still uses by default (KDE bug 527421). A fix has been proposed (MR !10173) but it isn't in 6.8.1 yet, and it will probably land in a later 6.8 release. If you're willing to test it, I can apply the patch to the openmamba kwin package now, after releasing the almost ready Plasma 6.8.1 update.