Also 6.12.12 didn't change a thing. But experiencing this since the
first 6.12 kernel, I suspected the NVidia-Driver being not ready for
6.12 kernels. Digging deeper I found a recommendation to install
nvidia-xconfig and run it, and what should I say: my second monitor is
recognized also with kernel 6.12.12!
So all nvidia users: install nvidia-xconfig and run it, after reboot
your nvidia drivers will work again!