[2021] Stellaris is using the integrated Intel GPU only instead of the dedicated NVidia GPU

It is nVidia's job to make that choice, so it is definitely odd that it only affects this one game.

You forced stellaris.exe to "high performance nVidia" option in Manage 3D settings, is that right?

nVidia driver and utilities all up to date with 2021 versions?  

That's not a common setup but I have seen it before. But I have not seen that borderless only runs on the Intel, that is really odd!  

I've seen this when the two video systems have different output sockets, is that the situation here? What kind of cable are you using?