There is definitely something to this.
I have been working with nVidia support to solve an issue with MSFS 2020 and the nVidia GPU Activity monitor that lives in the taskbar that can be enabled from the nVidia Control Panel. I had recently noted that it no longer showed MSFS as using my nVidia GPU and this was puzzling.
After reading your comments re: HAGS, I turned it off and now the nVidia GPU Activity monitor is back to showing that MSFS 2020 is using the GPU.
I don’t think there was ever a question that MSFS was not using the correct GPU, but it was really odd that the indicator would not work anymore.
There is something whacky going on with HAGS and MSFS.
Here’s the way it was looking until I turned off HAGS (nVidia GPU Activity is in the lower right corner):
And here is how it looks with HAGS off:
Note: Other applications would show up in the nVidia GPU Activity monitor if I set them to use that GPU, so it wasn’t a universal issue with HAGS. It was only affecting MSFS.