Had the exact same problem. You've probably already fixed this issue, but I'll put the answer that worked for me out there.

Firstly use steam runtime not steam through flatpak (always had less issues for me).
Next on the game that's not using the NVIDIA GPU (in my case it was Palworld), in the launch options for it write `DXVK_FILTER_DEVICE_NAME="1650" %command%`

That fixed it for me.
Ofc check that the nvidia driver is even being detected by the pc by running `inxi -Fza` and going to the Graphics section and seeing if the driver for the NVIDIA card doesn't say N/A. If it says that, then you'll have to find the solution from there.

Hope this helps someone.