Manjaro uses MHWD, so it will provide a default config, as mentioned, in /etc/X11/mhwd.d/nvidia.conf. That is useful because kscreen will generate its configs based on that.
Is known that past drivers created issues for some
But, in spite of that, you still need a config in /etc/X11/mhwd.d/nvidia.conf but you don’t have it, as you mentioned
The reason you need it, is to then create a fixed layout that then will sort also kscreen (once you delete old not working conf). Also, in some cases you need to enable early kms …