From NVIDIA Team:
This is not a limitation of the GPU itself as it is an internal scaling filter, using AI tensors, but each game applies it differently.
There are games that apply it natively and the graphics engine uses it for all aspects of the game, so in situations with multiple screens it is complete. But there are games that use it at the internal level of textures, resolution, or graphic pixels and it only applies in real time to the game’s main rendering, and other secondary aspects are adjusted by effects of Motion Blur, Filtering, or the same graphics engine, modifying textures and resolution that are not in the field of vision.
In the case of MFS, they use DLSS 3 at the level of the main camera, like VRRR Shading, and since their main monitor is the main camera for the game, it only applies in that visual field and the other visual gamut remains in the normal rendering of the graphics engine.
There are more games that use this type of use such as Red Dead Redemption 2, Elder Scrolls Online, or Forza 5.
This is because the implementation is designed for the graphics engine first and for the main camera, so setups with 2 or 3 monitors are not taken into account, since it is extra performance and very rarely used, to optimize in these types of configurations.
Therefore, it depends on each game and developer how they implement it and it is not necessary to return your GPU if it is something that does not matter too much to you in this case since we cannot control the way or implementation of DLSS 2, 3 or Ray Tracing and each developer adjusts it based on their desires, optimization or performance for the general public.